I saw a message a few week ago on a similar problem but could not find the post. Looks like I have a similar problem.
New Rebuilt Wide Ratio Toploader
Centerforce Dual Friction Clutch/PP/Throwout bearing/Roller pilot bearing
Modified Z bar linkage to clear my headers
11" refinished flywheel
New motor mounts
Vehicle has less then 50 miles on it. I was driving the vehicle in stop and go traffic. I had adjusted the clutchrod the night before and it was shifting great the next morning. After about an half hour my clutch pedal began to squeak another 1/2 hour and the vehicle would shake violently when you would engage the clutch in 1st gear. I could start up in 2nd gear with no shudder but first gear was Very Bad. Vehicle was hot but it did not geta any better after I drove it around and cooled it off.
I thought it had just came out of adjustment but everything was nice and tight.
I did notice what the squeaking sound was. My upper clutch rod (Pedal to top of Z-Bar) was rubbing the edge of the firewall hole. Something had shifted inboard but besides rubbing it did not seem to effect the clutch actuation.
Tonight I will check for a broken Motor or trans mount (doubt it). Anyone have any suggestions on what is the cause of this issue. I will repost tonight after I take it for a spin and see what it does witha cold engine.