Finally think I solved my vibration issue

vibration starts at 55mph and gets worse the faster you go. Light throttle or full throttle doesn't matter. Under deceleration its a different vibration but still just as bad. If I put it in neutral it almost goes away and comes back while reving (above 3000 rpm in neutral while traveling at 55+) Doesn't do it while reving in park.

1969 dodge dart 360 727 8 3/4 742 suregrip 3.91. stock cast crank, pioneer externally balanced harmonic balancer (pre 1992) 904 flexplate midwest converter weighted for external balance. dr diff drive shaft 1350 u joints. tried rear and angle from 4 deg down to 4 deg up no change. Tested with front wheels sitting on old wheels and rear on jack stands. wheels off, drums off, axles out, differenet 3rd members. new transmission mount, new tailshaft bushing. took belts off, factory balancer, swapped motor mounts. The only way i've gotten the vibration to go away is running it with a piece of wood on top of the jack lifting the trans up off the mount.