I've had clutch-judder problems on customer's cars and my own truck over the years and none of them were caused by excessive end-play. I'm not saying it can't happen, I just haven't seen it. The problems I've encountered were invariably caused by some clutch-related problem or torque-side motor mount.

One time it was a budget reman clutch assy supplied by a budget-minded customer. I charged her for the NAPA clutch and the labor to fix the problem and she's still sore about it 50 years later. On another job I slipped while holding the transmission and let it hang on the clutch disc and bent it. Had to do that job over again. In those days I still thought I could hold a truck transmission up without a jack. Another job went bad because I couldn't get the 10-3/4" 6-cylinder clutch and used an 11" V8 clutch disc. V8 discs have a different number of springs to accommodate the more frequent power pulses. When I put the correct disc in it the problem went away. On another job, the judder had been caused by the rivets that hold the diapharam spring to the cover working loose on one side of the diaphram. Another problem was because the disc was oil soaked. Another time the ball that clutch pivots was loose. On a Dodge dart it was a cracked bell-crank pivot. The list goes on and I'd look elsewhere before tearing into the newly rebuilt engine.

Not that this has any bearing on your situation but I'm sure mine has more than factory spec end-play. It judders slightly after several hours on the freeway. clutches can slip slightly and get hot running in high gear under load if the clamping force is not high enough. For me that's been more of a problem since I switched from 5.13 axle ratio to 4.57. I suspect the original 9-bolt clutch isn't up to the task.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.