I think there is WAY WAY too much time trying to make the linkage "perfect" when that is not the cause of the chatter issue at all. I have seen some really misaligned linkage before work perfectly my car included. It's inside the bellhousing, the flywheel, the plate or the disc is the issue here. Get the flywheel surfaced by a good machine shop then make sure the disc is not warped with a dial indicator and some type of lathe and a dial indicator.

As for solid motor mounts or motor plate if your stuff is balanced right there will be no severe vibration issues. Mine is smooth and I have gotten compliments on how smooth and quiet it is. Been running a motor plate and a poly transmission mount since I built my car about 30 years ago. The only clutch that chattered for me was a Ram sintered bronze 6 puck disc and a Hays race adjustable plate.

You have to start measuring from the crank flange then the flywheel to make sure you don't stack tolerances. Something is warped or bent somewhere.

Gus