My 1966 dart race car.

that is weird.... I just cannot see anything internal to that trans that would cause enough inertia to create a vibration as bad as you've explained.... other than a broken converter (I'm assuming it was pretty major from how far you tore the thing down again). You'd almost have to have a shaft "jumproping".... and I just can't see that happening. Now..... how could a guy bench test that??? So you changed the case and the output gear train and now it vibes..... like I said..... strange....