I'd be suspicious of the front bushing in the torque-tube...
These are notorious for going bad in Chevy cars and trucks with torque-tubes...
I had a '50 Chevy Fleetline sedan with a bad driveshaft bushing (4.10 rear too!)...the vibration would appear at about 40-45 mph, be at its worst around 50-55mph, and would begin to lessen around 60-65 mph (but then the 216 was getting very unhappy !). The vibration would also vary with slight changes in pressure on the accelerator.
My '54 Chevy 3/4 ton (open driveshaft) had a similar vibration that turned-out to be a bad rear bushing in the tranny...
At any rate, I think what you're feeling is a drive shaft vibration...check out "Oakie bushings" in our various Stovebolt vendors' catalogs and see what you find...
Good luck !
