69 Barracuda Clutch Shake Help Needed.

Sounds like your pretty much through the 9 yards on this clutch/shake deal Tim .
A couple outside the box things of note.
_In one 4 spd shake experience I found my pressure plate was off center in the clutch cover. It wasn't much either but evident to the eye if you were looking for it.
_I bought new from Chrysler DC/MP a new 10.5" B&B setup. In their chassis book they describe a fixture for checking clutch finger height. Basically a .310" disc with a 1.700" high standard. I already have a depth mic so I had the toolroom at work make the disc. Bolting the cover to the flywheel on the bench with the disc between, I found one finger almost .050" out and the other two within .004" of the 1.700" height specified in the book.
_One friend had a locating dowel missing for the block to bell alignment. Did you happen to check your run out of the bell?
_Any evidence of metal in the trans or rear lube.
_Maybe get the old plumb bob out and check for frame and axle square.