I have a strange vibration problem in my 57 Belair with Saginaw 3 speed. Vibration seems to only occur in 3rd gear. Feels like drive shaft vibration but is clutch or transmission related. New clutch, pressure plate, pilot bushing, throwout bearing, etc. At 50+ mph you can press the clutch pedal in and out 2-3 times and the vibration will stop until you have to press the clutch again. If I swap the transmission out using a loaner, the vibration improves dramatically. If I put the transmission in a different vehicle that vehicle inherits the vibration. Looking for ideas on how to solve this.