Methos-
Not quite that easy. I'm putting in different parts that came out, so I may have to re-shim the pinion to get it in the proper place. The IH rears had numbers stamped on the pinion shaft and case, and a process for shimming it, but I have to use the MTD manual trial-and-error process since I'm using the innards from an aluminum rear.
jbollis-
If you're building a rear for a standard GT (as in, NOT a super) and you're using a GT aluminum rear and a GT iron rear, you can leave the pinion shaft in there and just put the ring gear from the CI rear's carrier on the fine-spline carrier from the aluminum rear. You would still have to play with the carrier shims because you are putting a different carrier in the case. It can't fully set itself, because the side-to-side location of the carrier will change the contact pattern.
I was going to use the original pinion shaft, but I can't since the ring gear that goes with it has a crack in it. I don't know if this is just the aluminum rears in the super, or if it goes for all aluminum rears, but the driven reduction gear is 0.100" thicker than the same gear in the iron case, so I'm going to have to pick up some shims to get that where it belongs.
You can use
this manual for the iron rears and
this manual for the aluminum ones. They disassemble differently.