I dont know your reason for changing but did you know your GM rear takes less power to turn than the 9inch ford? Dont get me wrong i use 9's in most cars but only because of unsprung weight and there cheap
We have figured out in racing the best way to get both worlds....they now make a center section for the 9inch to run GM gears....yep i aint BSin. Its around a 7-11% loss with a 9 depending on whos dyo sheets you look at(plus another 9% with a quick change). Back to your question you can have the housing cut and center the yoke or cheat and move the tranny a bit and just leave the rear offset. It does not seem to eat u-joints and i think i like em centered but there are a h ell of a lot of them running offset around out there with no problems.