Food for thought, I just got rebuilding a 69 GTS Dart for a customer. Originally it had a 383 and a 4 speed but he changed it to a 440 in the late 70's. Anyway when I got the car finished and started to test drive it it had a horrendous clutch chatter. It chattered so bad it was hardly driveable because you couldn't let the clutch out in first gear. I tried three different clutches with no improvement finally I noticed the pinion on the rear end was pointing up. I measured the pinion angle and it was 1 degree up in relation to the trans output shaft. I got some 5 degree wedge shims and changed the pinion angle to 4 degrees down and instantly the problem was solved. Now the clutch is as smooth as silk with no chatter at all. Something to double check possibly.