Persistant vibes..

1964 Dart
'When I first got it, 50/65mph rhythmic vibes. Lightens a bit when I let off the gas.
new tires, balanced, still there.
u-joint conversion, still there.
/6 to v8 swap with new tranny, still there.
rear axle swap, still there.

Unless I'm blind, what the heck am I missing? There is literally nothing left original in the driveline and the vibes have never left...

Ideas?