Okeh, that's not normal, even with stiff mounts. How certain are you that you got everything lined up and centred correctly, with no metal-to-metal contact anywhere? How certain are you that all the nuts are torqued down correctly with all applicable washers in place? If the threads end before the assembly is firmly secured, you'll reach the torque, but the engine will still jump up and down, creating the jackhammer effect you describe. Also remember the third mount in the system, the trans mount; by installing nice new engine mounts, you might have created a situation that was masked by all three mounts having sagged or rotted or etc.

I'm especially suspicious because yeah, the 60-durometer mounts are the softer ones, same ones I used without anything like a jackhammer effect.
Maybe or not. If there's a metal/metal contact issue you inadvertently correct in the process of re-replacing the mounts, you might erroneously conclude the cow-poo mounts were the fix.

OBTW, because I'm a writer, that thing about "durometer" kept bugging me until I figured out—late last night—what the best analogy is: "gauge". As in, "use 10-gauge wire" or "it's made of 20-gauge steel".

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

Somewhere between "Very" and "Completely."
I've got a lift. I did this with the car up in the air. I looked at everything when I was done.
See above.
There are four engine block screws with washers (all original), two studs on the mounts that screw to the frame with new Nyloc nuts, each with a washer, and three studs on the mounts that attach to the engine brackets new Nyloc nuts, each with a washer.
Everything is run up tight by hand, final tightening with a 14" breaker bar.
Changed that too.
True. But it would be fixed, I would have the fancy new mounts if I wanted to try re-swapping them, and I would have the knowledge and the system for changing them fairly expeditiously.

I will admit that the driver's side mount was a bit bitchy, wanting very badly to rotate, so as not to be in line with the K-frame member, and I had to hold it in position with a pry bar as I tightened it to keep it straight-ish. The mount has one stud up to the engine bracket and one stud down to the frame, and they are not aligned, so they seem to want to twist the mount as the engine is snugged down. Everything else fell into place as though it had been made for it, which is was.
Can't argue with the analogy, but "gauge" is a very old usage (Old Germanic languages, "Pole" [thus to modern English "Gallows"], then "measuring pole," then "device used for measuring," then "the act of measuring," also reference to the fact that a thing is consistent with a measurement standard [railroad gauge, wire gauge, sheet metal gauge, shotgun gauge], then a mechanical device used to make accurate measurements), well established in the language separately from its technical meaning (to gauge someone's character), while "Durometer" is not.

"Shore Hardness," or "Shore Scale A," on the other hand...

– Eric

Awright. Just chikkin'.
Very fine. And are we sure the washers are thick enough to overlap the threads adequately to prevent bottom-out?
Good, but not responsive to the question at hand.
Good; what kind did you use?
Good… …but
Surely. How old must a usage be before it is considered legitimate, please?

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

They're the ones that came with the new parts, and they appeared to be appropriately sized.
Fair.
Everything went together, looked tight, and felt tight, with no apparent gaps or misalignements.
Polyurethane from Imperial Services. PolyBushings didn't list one, and I wasn't sure whether the one-stud or two-stud distinction mattered, and they listed one specifically for my year, so I picked them.
In fact, one-stud will fit anything, two-stud will fit two-stud, but not one-stud applications.
My car had two studs.
Pre-21st Century, unless it refers to something that did not exist, and so could not be referred to, before the 21st Century.

– Eric

Just a thought:

It is possible with the poly bushings that they are a skosh taller / less saggy than what they replaced. It is possible that the trans tail shaft could be contacting the top of the cross member - My '62 and the '64 had a small rubber bumper in the cross member hump for the top of the tail shaft to transmit less NVH.. You might be hitting it with the new mounts installed.

Greg

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I will admit that I did not actually stick my fingers up in there to check that clearance, but visually, the tailshaft was in the same place before and after.

The engine itself was not. You had pretty much enough room to slip a sheet of paper between the oil pan and the K-member before the change, something like 3/4" afterward. The old motor mounts were completely shot.

Incidentally, I was just out there a little while ago, and I tapped on the motor mounts with a sledgehammer, and could detect no looseness, and could not duplicate the sound.

Thanks for the advice!

– Eric

Okay, Poly mounts replaced with Pioneer mounts (motor mounts Made in China, transmission mount Made in India).

Harsh rapping feeling (it was never a sound) at idle went away with new (cheap) motor mounts.

Rough, buzzing feeling in the seat of my pants went away with new (cheap) transmission mount.

I didn't try to get scientific, like by pressing a ballpoint pen into the rubber (which is essentially the way that the Shore Hardness is determined), as it was about 100°F in the barn and I was in no mood, but the rubber on both the Poly and the new foreign mounts felt pretty much exactly the same when pressed hard with my finger. The rubber on the 60 year old original mounts felt significantly softer.

On disassembly, I could find no looseness, malpositioning, or assembly error that would explain my symptoms, and the new mounts appear to sit exactly the same way that the Poly mounts did.

Height of the oil pan above the crossmember is the same with the new and the Poly mounts, about 1/2" (the bottom of the pan sits at a slight angle, so the measurement varies across the pan). With the original mounts the space was paper-thin.

I intend to keep the Poly mounts for a while, to be sure everything is working well, but I will likely want to sell them off to someone else at a substantial discount from the original (eye-watering) price in the near future.
If anyone is interested, please let me know. They have my ringing endorsement.

– Eric

ps: yes, there is a small rubber bumper above the end of the transmission tail section, and yes, there was and is clearance between the two when assembled.