I have seen this many times.
I believe in a lot of cases the difference between cold not grinding and hot grinding is the temp of the oil in the transmission.
Clearly the clutch is dragging.
In a lot of cases I have found that the problem is the clutch disc is not running true.
I just went through this on a 1941 PU and the new disc had close to 1/8" run-out.
It is always a good idea to slide the disc on the input shaft of the transmission and give it a spin to verify that it runs true.
Dave,
Great idea about checking the disk run out, but kind of late in my current situation. What can I do to remedy the situation in the short term? I had already planned to take out the transmission for a rebuild during the dark days of winter but wanted to enjoy the truck this fall...I have two events that I was much looking forward to being present. Can I visually check for disc run out if the truck is running with the clutch fully engaged? Can I put some thick 140 weight gear oil in the transmission to alleviate the symptoms?
It just runs and drives so darn good otherwise, this is killing me.
Paul