Engine/drivetrain vibration?

Is this vibration RPM generated or speed generated, take the car down the highway till ya feel the vibration and put the tranny in neutral and let engine idle down.
If it's RPM I'd start at the beginning by first determining whether the crank is forged or cast. There is no guarantee that the original balancer was correct before you had the engine rebuilt. Once you've determined the crank for sure then work your way outwards to the balancer, flex plate and converter.
If it's speed related I'd check that both u joints are installed correctly and not worn out, as previously mentioned, I've had u joints starting to go and first place I felt them was when you're sitting in the drivers seat.

Terry