My 2 cents- Your clutch disc may be running out of splines on the input shaft, a buddy had to elongate the splines on his swap. I'd assume this is on and old Chevy truck the year doesn't matter; you need to go back and review what you did and compare the NEW trans input shaft/throwout brg components with the original. i.e. check input shaft/spline length etc.
Best of luck, hang in there you'll find the rub.
