Okay, I have a 66 T-Bird with a cruise. I bought a 64 t-bird parts car that was wrecked. Started it but could not drive it, But it sounded fine and the was told everything was good. So i changed the shit lever in the trans to the 66 style and installed it. Everything went together very smooth, no forcing. The trans shifts great but i have a little vibration at about 42 while applying slight pressure to the gas pedal. Was not there with the old trans. I could detect this vibration in park while slowy reving the engine to a certain point. I believe the vibration is foward of the drive shaft elimnating that and the rear. While taking the parts car trans out it did drop and land on the tail shaft from about 2 feet up but no cracks or anything. So any ideas what the vibration is? Also will it hurt not to worry about it and just leave it? Thanks Bob
Make sure the mounts have not sagged and rubbing metal to metal.
Pay particular attention to the engine mounts.
If the output shaft was bent the vibration would be noticeable when the vehicle was moving.
A vibration at rest is like you said, ahead of the driveshaft.