Have any flex in your motor mounts? Years ago, my 70 Road Runner motor mounts had a little flex in them (new at the time), I found that when I dumped the clutch, the motor was rolling over enough for the clutch linkage to slightly disengage the clutch causing a slip.

Back then, I installed a turn buckle between the motor and the K member to eliminate the motor mount flex and the slipping clutch went away for a couple months of abuse, before the clutch was dead. Tires back then didn't have much grip, like tires today have, but neither did the clutches. Gene