I would cut the old spring perches off and use new ones, set them on the spring perches, install the U bolts, mock it up so you can rotate the rear end housing and then set pinion angle to where you want it, tack weld the perches to hold them and then reove the rear end so you can weld it on a bench. Take your time on the welding, weld a inch one side and then switch to the other side, do one to one and half inches at a time so you don't warp the axle tubes, brace the perches also