the 3 sp doesn't need a computer. It's still the old mechanical (or whatever!) design, not electronic like the 4sp. Just take out the computer for the 4 sp and it should be all good. You might have to put in the computer from the gl if you still have problems, as long as they're both MPI cars. It is actually cheaper to put a 3sp in place of a 4sp istead of straight replacement i have heard. If you go and ask a tranny place for their idea's on changing trannys, they will be able to tell you what has to be done. That's what I believe to be true on this topic, others on here may know more...
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)