[geeteeohguy]

First, I would determine if it was the engine or drivetrain, using the techniques already posted. Personally, my '65 had a mirror-blurring vibration for years. Turned out to be a worn front driveshaft yoke. A new yoke and bushing in the muncie fixed it. I have also run into cars where the locating tang was hammered/worn on the rear yoke, making it nearly impossible to center the driveshaft. My gut tells me you have a driveline issue. Also, check the weights on the rear brake drums, etc. A run-out gauge will help you check driveshaft run-out and wheel run-out, which is also not a bad idea. I feel your pain....vibration is a fun killer.