I've got a '71 vintage 350 Chevy in my deuce sedan and I've got a vibration that starts about 1800 rpm. The engine was professionally rebuilt and supposedly all of the rotating assembly was assembled with matched (by weight and dimension) but not dynamically balanced. It is definitely rpm related (not vehicle speed). The flywheel, the clutch and transmission have been changed with no effect on the vibration. I'm thinking I may have a bad harmonic balancer. I don't know for sure but I doubt the balancer was replaced with a new unit in the rebuild. Any ideas? Other possible gremlins I haven't thought of?