rear tires not centered in body

IMO. Your quarters are hung a little off. Your spring perch is correct. Your reveal to tire to spring is the same on both sides. Frame rails are straight to each other. Front spring anchors could be off or quarters are off. Do you really think at the factory in the 60's much was given to detail. All these cars came with small tires so who cared about a 1/2" on reveal that was huge with factory tires. On my 67 fb fish the quarters are off by a 1/2" and the front spring plate is off a 1/4". This would never have been picked up till i put 15 x 8 rims and 275/60/15 MT cheater slicks.