I've a 67 w/289/Borg T10 and have built the car from pretty much a junkyard nightmare. Have owned the car for over 25yrs and is very nice (if I do say so myself). A few years back the throw out bearing went out so I replaced it and the clutch/pressure plate with a new 10.5 in (original was 10in). Now I seldom drive the car but somewhere back, I started to notice a slight vibration at about 50mph....a rythmatic sound...repeating itself...the higher the speed the more frequent the vibration pattern.
The car came to me with a 3spd manual. I used the same flywheel for the 4spd Borg and (at that time) used a 10in clutch/pressure plate. As I said, later I replaced the bearing, clutch and pressure with a heavy duty 10.5in version. I had the flywheel resurfaced at that time. Later (not initially) I started noticing the vibrations.
I'm wondering if there is a difference in a 3sp and 4spd (157tooth) flywheel, AND/OR if there is yet another difference in a flywheel suited for a 10in clutch/plate and a 10.5in clutch/plate. Could my vibration have developed (later) from not having the correct (maybe heavier) flywheel when using the 10.5in clutch and/or 4sp tranny?
PS....it's not the driveshaft, nor the U joints, nor harmonic balancer, nor anything that is belt driven on the engine. Engine revs and runs (when in neutral or with clutch pressure plate depressed) without vibration. This vibration occurs when traveling down the road, starting about about 50mph or higher.....sounds and feels like it is from the engine back.....oh...have also changed diff chunks around...so it's not there either.