As a heads up last year I was thinking this same thing was happening to me, felt my clutch getting gummy until it finally "went". So I got to business after towing it home pushing it up in the drive and started wrenching went down got a new clutch and pressure plate assy. Pulled mine which still looked pristine no broken teeth, clutch was not burnt, a little mystified by this.
I let that night come and go. The next day I tended to a couple other issues at hand and put my engine back in.
Started right up rolled out of the drive, engaged the clutch and went nowhere. I was baffled, come to find out I had a loose rear brake drum (right hand side) and it was gradually 'slipping'. Turns out that slipping was my drum hopping, or machining itself. Until that fateful day, it machined itself smooth. Good news I fixed somethings, got a new clutch and. . . A new old brake drum. Tightened the hell out of my rear axel nuts.
So this wouldn't happen again.
Btw this is how it should look. (That is if your early is a wide five)
Just a heads up. Been there did that.
-Eddie