Without me rereading the entire thread, did your truck ever run ok prior to the clutch slipping?
Are all of the major parts still the same...engine, bell housing, flywheel and transmission.
Was the trans input shaft ever taken out?
Like Kevin already mentioned, put a straight edge across the bell housing opening and measure to the pilot bushing. Them measure from the face of the trans to the front end of the shaft splines...what are those measurements?
Put the trans in without the clutch disc and see if it slides right up to the bell housing with out using bolts...it should.
Mike B
