vibration
Yeah, you never know with rebuilds.........some guys just plain don't understand the internal/external variations on some of these motors, and cross parts that won't match......result, vibration.
I don't know of a 318 that was internally balanced........340's yes.......but those parts mix and match something fierce, and they get crossed up all the time and cause this problem.
My guess is that there's a mismatch on the harmonic and/or converter.......but you have to know what they did with the motor to know what to do with the externals.