Had the same vibration at the same RPM! It was engine RPM, not drivetrain that changed the vibration.
1. Found the flywheel I took off had a series of holes drilled on one side. I did not pay any attention. It came off the 216 that was in the car and installed on a 235.
2. Vibration destroyed the rear main bearings.
3. Found also that the crank end play way out!Thrust surface of the main bearings was shot out or not right! Crank was walking back and forth over 3/16 in.! End play should be .008 or less!
4. This was a fresh rebuilt 235.(Not by me!)
5. New crank, bearings, oil pump, and a different flywheel (from a known 235, no drilled holes!) NEW clutch cover and plate, fixed the problem.
6. 235's are internally balanced. Harmonic balancer and flywheel are netural.
