The 360 converter is balanced differently than the 318/340 converters. The later 340 with the cast crank is also externally balanced and uses a different converter than the early 318/340 internally balanced engines.
Thanks guys I didn't even think about that as to the reason for the vibration in the car. Going to a 4 spd anyway, will make sure I have a 360 flywheel though. Craig got the pilot bearing today did you get my pm.