I read your page, I'm impressed with the history. I'm a rookie. Ok just for clarification, with the clutch pressed in the RPM'S drop becuase the engine power is not engaged with the tranmission. Technically the drive shaft is still turning though. So if the vibration is still there, more than likely it is the drive shaft?
Man I've got a lot to learn.
right, push in the clutch, driveshaft, trans, and clutch disk are still turning fast, flywheel, pressure plate & mtor slow down.
Could also try it in neutral, clutch out, this would rule out the clutch disk.
The trans itself is most likely not the problem, tho if the rear mount is separated, that could be it. You could try to lever the trans off the crossmember with a prybar to see if the mount is separated.
I'd still lean toward the driveshaft. With the car securely on stands, you could have someone run it up to speed while you watch it, just be careful.
I really need top update my webpage, it's 2 years old, since then, engine rebuild and stroked to 383, tko 500, last summers run to the rockies, and now she's down for new rear end, some floor work, 4 wheel disks, and a new front suspension.