70 mustang clutch chutter/chatter

I would start by making sure your freeplay is adjusted to the right distance and then make sure your bellhousing is snug to your engine and that your tranny is snug to your bellhousing. I had this problem with my car when I first got it. Then I had someone push on the clutch pedal while I looked underneath, this is when i noticed the tranny separating slightly from the bellhousing as they pushed. Upon further inspection I noticed I was missing a tranny to bellhousing bolt, one was stripped and one was loose and one was o.k.