Lawson,
Before doing anything drastic, check you have free play on the clutch pedal linkage. It may be that the clutch has worn a bit and the fingers have come out slightly and decreased the clearance. There will be an adjustable link rod adjacnt to the clutch arm on bell housing (not too familiar with GMC, but you should see it).
Also, I agree with Tony (N.O.S.), except on one thing, always use an aligner to set the clutch plate centrally, without it you will struggle. True it is possible to do without, but then you have to view the plate end on and if you are only going to pull the gearbox back about 6 inches, then eyeing it up will be impossible.
Richard