I think a gbody rearend is 58" flange to flange not 100% sure though. I personally would just buy a bolt in housing with axles but if you have the capabilities to weld on brackets in the exact location as the originals then I suppose you could attempt it, I would also check the axle tubes for straightness on your what I would assume is a used 9" rear.
The aftermarket rearends are made in a jig so that everything is welded in precise locations if you can't get it exact you could be fighting suspension issues all the time.