Clutch Chatter

[70TA]

I checked u joints, I wondered about bell housing but it's the original one and if it's off it would when car was new this is a 55,000 mile car.

[Rixls6]

Had the same problem with a 69 Chevelle I owned.
The new owner bought it knowing that it had clutch chatter, then chased the problem in a similar fashion as you.
One thing that helped was to replace all the rear control arm bushings.
Not sure if anything else was done after that.