Highway speed vibration ('57 Thunderbird)

[yblock57]

Getting a bad vibration starting around 55 mph that gets worse up to 65 then gets less intense at 70+, but still there. Very annoying. Feel it in the seat, steering wheel — whole car. Tires are 4 yr old B.F. Goodrich Silvertown bias-plys. Just had all 4 rebalanced. Rims are true. Tires didn't show any signs of trouble on the machine. Run the exact same tire on the '57 Fairlane with no issues, so doubting that's the trouble.

Replaced:
— shocks
— universal joints
— trans mount
— engine steady rests

Driveshaft does not have any weights. Would y'all have that checked next? Being so short I didn't think that would be an issue, but I want to get this thing cruising smooth. Car has not been on an alignment rack in over 40 years, but nothing has been changed/messed with in the front end. Could use A-arm bushings. Wheel is off center but doesn't pull. Very few miles put on it until recently.

Thoughts?