It's great fun getting back up and running! Thanks to you and all that contribute here for the opportunity.

A little update: I've set the valve lash (hot, off) and adjusted the idle mixture screw and double checked my lash with the engine off & hot once more. Then, I tried to start it back up and it just wouldn't idle at all. Instant stall. Only way to keep it running was by giving it gas.
Gut reaction thought "vacuum leak" so I kept my finger over the PCV-to-carb hose and gave it another go. Started right up, idling just fine. So it was too lean because of open PCV hose.
I put the valve cover back on and hooked up all the top-end goodies, and - just - awesome. Running like a top at ~650 rpm. Best it has EVER ran in my possession.
No smoke out the tailpipe, no shaking imbalances!

So, I took her around the block and it ran up the nearby hilly road flawlessly. On the way down, I gave it a fair bit of gas and coasted down the road back towards home, and it was just after letting go of the gas that I lost power steering and realized the engine stopped.
Parked on the side of the road, and put gear into park. I tried cranking, and on the times it did crank (see below regarding starter issue), it just wouldn't start the engine.
Temp was fine and idiot light wasn't lit when it shut off. I verified spark and fuel were good. Eventually, I did get it running for about a minute after many attempts, but it was quite rough and eventually stalled out.

Got a couple guys to help push her back into the driveway, waited about 10 minutes, and tried again. The engine started back up relatively easily and continued running without stalling - but it was a bit rough. The cabin light was flickering and the alternator needle on the dash was shaking some (my indicator of smooth vs rough running) and so was the engine more than it was before driving... Definitely misfiring once again.
Any ideas on what to check? I'm stumped

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Regarding the starter, it has been giving up slowly yet very surely and would only crank the engine about 1 every 10 tries. The rest of the time it'd just give a heavy clunk when turning the key but not spin at all. Battery is fully charged. Connections are good. I've ordered an ACDELCO 3361042 from RockAuto as a replacement, arriving in a couple days.

Still there's progress... Good job.

Could be lots of things:
- choke sticking on
- valves still not set right and getting tighter as it heats up
- still a vacuum leak somewhere or however you plugged your PCV didn't work or the PCV is bad/going

Lou

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Home of Slant6-powered fun machines since 1988

The PCV valve is new (at Dan's recommendation) from a few months ago.
Choke is fully disengaged when warm, and I just checked the valve lash hot while running a few times across. Some of the exhaust valves were too tight and I loosened them up. I plugged the PCV hose once again now that I have the valve cover off, but it's still not as great as it was before taking it for a spin.
All pushrods are still in place (phew!).
I held a rag over the carb opening to see if perhaps it's running too lean, but it would get worse and begin to stall out until I remove the rag.

It was running great until I gave it some heat, and now it's running rough. It idles without stalling, but shaky. Video

With the engine running as fast as it is in the video I would think you should be slinging oil everywhere! I don't see it much at all?

Simple things. I'd pull the plugs and look at them. Make sure they all look ok. Does one look different than all the rest? Maybe a piece of carbon got into one of the electrodes? Slow the idle way down and pull the plug wires off each plug and see if one of them makes less difference when you pull it off. Zero in on that cylinder.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

That was at curb idle, I think set to around 700-750. Here's a video from before the drive at slightly higher RPMs. It's definitely getting decent pressure there. Should it be even more than this?
The original video isn't the best quality and it's hard to see but there was definitely some oil running down the push rods and covering the flat tops of each valve spring. Definitely not splashing about like this video above, though.
I made sure to clean out the rocker shaft and each arm's holes well before re-assembly. Could it use a servicing of the oil pressure relief valve?
Here are the plugs, in cylinder order 6-5-4-3-2-1.
Nothing that appears immediately alarming from what I can tell
(I'll put them back in without the washers)
I suppose I'll put them back in and do the plug pull test which should still work to narrow down which cylinder is misfiring for other reasons

Those look OK. I would not mess with them. Do the plug wire pull test and see about which cyl(s) are weak/dead.
Lou

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Home of Slant6-powered fun machines since 1988

Sounds like it might be fuel delivery or ign timing at this point? What is your ignition timing set to (vac adv disconnected)? What is the condition of your distributor and cap/rotor?

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Home of Slant6-powered fun machines since 1988

I see a few things about these plugs:

1. The metal ring washers are on them. They shouldn't be, on a '63-'74 head. Details here.

2. They're NGK № 4s. That is a hotter-than-spec plug; where'd the idea come from to install these? If your mixture were proper and there weren't any other problems, these should be bone-white. This suggests you're running too rich across the board. The ring washers that shouldn't be there make the plugs run hotter than they should, which adds weight to the apparent overly-rich condition.

You'll want to put some attention on the carburetor and the other tune-up points. Tune-up parts and technique suggestions in this post. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

Then put in a set of NGK ZFR5N (stock № 3459) without the metal ring washers.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The stumbling increased for each one that I pulled. Admittedly, I had to run at the end and did not have a chance to pull the 1st cylinder's plug. So it does not seem to be an issue with any individual cylinders (unless that 1st one is the culprit. I'll check tomorrow for certain.).
I haven't got access to a timing light at the moment, but the cap & rotor is new and still appears clean and undamaged from the underside.
These're the plugs that came with the head, and being that it came from a fully working engine, I was under the assumption it'd be alright - and it was until my little drive.
Yeah... recalling now, I took a picture. Here's how they looked before going into the engine: Image

I ran it again today and it's acting very unpredictable. It shakes the most on idle but when I put it in gear, it feels fine. I drove it 100ft but then stalled, as well as a few more times on the way turning around and back. I didn't have my tach reading at that moment but it couldn't have been less than 650rpm at idle. A few minutes later I ran it again, and it ran smoother on idle in Park, and then after around 10 seconds the alternator needle gauge started hopping around again. Choke is wide open throughout all tests.
There is a heavy smell of fuel coming from the exhaust, and getting too close stings the eyes.
I did a full carb rebuild not too long ago, and the needle-idle-airway and other passageways appear unobstructed now too. (Also installed a brand new fuel tank at the time).
I played around adjusting the idle mixture again today as per this thread, and not much difference in the misfiring. It starts to choke up if the screw is turned in too much.

Go through your fuel system possibilities for excess fuel, starting with the fuel pump (pressure), then the needle and seat, then the float, etc...

– Eric

Those plugs should be just fine for what you are trying to do and will not affect anything significantly. I suppose you could try a new set of those or the ones Dan mentions, but those ran fine in my engine in mixed street/hwy driving and racing. They should not be worn out.

I'm still thinking mixture/carb problems. You need to measure timing before you go much further. You could also loosen all the valves by 0.002 or 0.004" and see if that makes a difference.

Lou

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Home of Slant6-powered fun machines since 1988

Did some more testing. Still no timing light, but perhaps my vacuum reading can substitute.

Fuel pump: 5psi
Float level (wet): Image (I don't exactly understand how to read this. Too high or too low? Indicating lean or rich?). FSM spec is 27/32. Tested from fuel level to machined carb surface - where the top "T" bar is the surface. Tested via removing economizer diaphragm.

I did another test prior to testing float level: I discontinued the fuel line to carb inlet and plugged the hose leaving the pump. I started the engine to allow the bowl to drain to see if perhaps it was overfilling to which the stability would improve as it drinks. There wasn't much of a difference.

Idle vacuum reading (from PCV carb inlet, warm engine): very steady needle at 20Hg @ ~650rpm
In gear that drops and hovers around 16-17Hg and engine dips to around 450rpm.

It doesn't run terrible, but engine is still shaky when not in gear (and RPM shakiness can be heard and seen visually by wobbly alternator gauge needle + consistently flickering interior "map" light).

If it were me, I'd start by raising the idle speed to 800-850 and see if you like how runs better. Our stock 68 Dart never idled super smoothly at below 800 RPM. This is a 60 year old vehicle/engine... If you don't know your timing at idle with vac adv disconnected, then you are flying blind. Get a dialback timing light and use it. OR, you could try retarding the timing (rotate dist body clockwise) by a few degrees (move slotted plate about 1/16 - 1/8" relative to the 7/16" bolt head holding it down) and see if it smooths out. If worse, rotate the other way and see if better.

Lou

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Home of Slant6-powered fun machines since 1988

Got it. Will give it a shot.
I may have too high of expectations, as you mention, it is an old engine.

I would like to say - the fact that we've gotten this far is incredible.
Thank you, Lou, for providing me with the replacement head and all of the advice thus far. It's a much much happier piece of machinery running on all 6 cylinders. Only 3 head gaskets later
SlantSixDan - the Kroil method must have worked to free up the rings, because there is zero smoke coming from the exhaust any longer. Splendid! (Kroil earned its new place on the shelf, in place of "bREAkS-LOOSE-FASTER!!!" PB Blaster).
And Rick and the others who've assisted with their wisdom along the way. I really do appreciate it.

I will keep updated

Martin