I agree. A #1 place to look is the motor and transmission mounts. If those check out then ensure that the motor is firing evenly (complete tune-up with new parts and a fresh carb balancing).

If it is still wonky I would go over my build notes. Was anything machined? Were machined parts balanced after their work? Did all the parts go back into their proper positions the correct way 'round? Were any of the reciprocating/rotating parts (other than the cam) new? Were they name brand or Volvo?

This will give you a list of suspects to check. I would start on the outside and work my way in (clutch and flywheel first, etc.).

Good luck, as you may have noticed from the postings nasty vibrations seem to be a common malady. In that vein, does this occur at idle/rest or while rollin' along? There could be an issue unrelated to the engine.

Mike!