69 Barracuda Clutch Shake Help Needed.

The block to Bell, yes...but not so much like what you describe, Misalignment Block to Bell can cause bad wear or breakage.....Im not so sure how accurate the factory was in aligning the block to bell?
Good luck, hope you find the issue! Let us know what it was if you do. Lots of good ideas mentioned here, pick the cheap and easy ones first, I do...cause I'm cheap..lol