Cujo8,
The 54 rear axle for my Chevy is 62 1/4 inches flange to flange and the ratio is 3.9 to one. I am not sure if the GMCs are the same - probably. Your 57 pass rear end width sounds like it is too narrow. You might think about changing the gear set to a 3.55 to one, that is if you want to keep the stock closed drive shaft.
Others here have done the Blazer swap so they would know more. Find an early Blazer with the ratio you want, order a spring mounting kit from one of the venders. "No Limit"
www.nolimit.net has the offset axle mount kit for $103. that relocates the axle to the center of the wheel well. I am not pushing their products, because I have not used them, but the kid across the street from me works there and he just gave me a new catalog yesterday. Nice catalog. Get the Blazer drive shaft with the rear end, it is probably too short but you can salvage the ends. Find a good drive shaft builder. Someone else will no doubt direct you on the brake cables.
If you have a closed drive line now, you will have to change the trans to an open drive line, which is probably why you want to change the rear end.
Get dirty, bust some knuckles, and have fun.
