rear tires not centered in body

If one side fits and the other doesn't then correcting the stress on the springs by moving the perches should make it tighter on both sides. This then requires trimming the inside of the lips which generally have about 3/4" to play with. Then there is also the possibility of rolling the lip with a bat or tool made for the process. Because one side is off just means that forcing the B body rear on the narrow spring mounting is pushing to the side with the most play or wear in the bushings. You could loosen the U bolts and see if you can center it a little better but I would remove the stress. If your wheels are 5 on 4" pattern this is not a B body rear axle though.