Whatever rearend you use will have to be (narrowed). If you have the 81/2" GM unit it is probably worth the cost to narrow.
You will need to know just how much you can move the ends of the axles (in) befor they will allow the tires to rub the springs or the inside of the fender wells. This is not a (cheap) thing to do & if your axles don't have enough meat on them, you will have to buy new ones from someone that makes custom axles.
I just did this to a 9" lincoln with disk brakes & it is a 2000.00 opporation at least! to go this way. you can get the housing narrowed for around 200.00 & THEN! the fun starts! Axles = 400.00 you might be able to get a simple narrow job done without changing carriors (posi) or anything for a grand.
Make no mistake! get someone (qualified!!!!!) to do the job that has the (Jigs) to set the rearend back up! This CANNOT! be done in your buddies garage! If that housing isn't welded back together PERFECTLY!!!!!! you will loose axle bearings right & left! & probably have some NASTY! Vibs to boot!
why would you have to shorten the alxe? how wide are the wheels your buying? if anything if your going with wide rims, you would have to tub the frame and the bed to accomidate the wide rims. the wheels would could just have alot of backspacing on them so from the front they wouldnt look that deep, thats all.