To get those numbers I had to bust out the Law of Cosines and the Quadratic Formula. My diagram is below; if you want the algebra I can share that too.

Wow! You are way to smart fer me! I have all I can do just to plug the numbers into one of many online calculators!!!

The program I used spat out a DCR of 7.9, (everything assumed your numbers are spot on), Josh is right you have to be careful on the
DCR with short cams like this as they don't bleed down like the bigger sticks, the compression test would give us a good picture of what is
going on in there...

That being said a few other thoughts here:

1) After break in did you properly lash the valves?

2) I would rebuild the carb, if it's a holley 1945, I would set the linkage to the 1976 settings and make sure the main jet was a #61 (these are the same as Holley 2300/4160 jets)
The 1974 OEM carb is horribly lean and runs a #58 jet, once the jet is changed and the linkage set to handle it, it should drive better and not lean out under load.

3) What vacuum accessories did you hook up under the hood? OSAC???? Just the EGR? (The pics look like you installed the vacuum amplifier and all the factory spaghetti to the
emissions controls).

4) Something is not right with your mechanical springs, they are too light, the 1974 375504x distributor usually plateaus in the mid 20's crank degrees and doesn't give you the last
4-5 degrees of advance unless the engine was at 5200 rpm... sadly with a heavy A-body and the 2.71 heavy duty rear axle with an automatic and stock convertor
the mechanical curve needs to be a bit slower than what you have (or swap to 3.21's...LOL...to match that quick advance). I would suspect that one of the springs is not tensioning correctly,
or has broken,or some springs got swapped in it's past... Doing what Lou would help "I'd get a recurve kit from Charles Brooks and shorten the mech adv slots and use diff mech adv springs",
or ordering a 3874598 1976 (Federal) slant six distributor would be a plug and play (set to 8 BTDC), if not wanting to pull apart the old unit.

Good luck.

-D.Idiot

You did a fine job of collecting and reporting data.
Did the engine get into pinging when you did the test with the vacuum gauge connected and the advance cab disconnected?

Are you running regular gas, 87 octane.

A cold air intake can also help to reduce pinging. Reducing under hood engine temps by running a 180 degree thermostat can also help. I have both of those on the 83 D150.

So try it without vacuum advance, if that is successful hook up the VA and try a tank of higher octane gas. If that is successful, no pinging, figure out a cold air intake and then dial in a few more degrees of initial advance.

I know the air cleaner has the heating system to provide 100 degree air to the carburetor, but my underhood temperature is actually much hotter than it was before I rebuilt the engine. That's because I filled the cooling system with waterless coolant. After a long drive yesterday and a few minutes of idling in the driveway, the block was 250 degrees (measured with infrared thermometer- radiator was 220 degrees).

I wonder if I need to switch back to regular coolant.

I'm running a 195 thermostat and I could swap in a 180, but I'm not sure how much difference that would make

2) I don't know what vintage 1945 I have or what jet is in there.

Seems like this is an obvious place to start. If it's too lean, messing about with the timing and everything else isn't going to be very effective.

And before the rebuild, the block measured out at what temperature (in the same spot, after the same amount of runtime, measured with the same thermometer)?

What spark plugs are you running?

I never carefully noted the number. Because it was always close to the thermostat design temp, I just assumed the thermostat was working as designed.

I would rebuild the carb, if it's a holley 1945, I would set the linkage to the 1976 settings and make sure the main jet was a #61 (these are the same as Holley 2300/4160 jets)
The 1974 OEM carb is horribly lean and runs a #58 jet, once the jet is changed and the linkage set to handle it, it should drive better and not lean out under load.