A body 8 3/4 are actually 52 3/8, not 51.400 like that chart that's floating around states, IIRC.
A body perches are 43". 62-70 B rear is 44" so they are 1/2 inch wider on each side. Lots of people just pull the springs to get them on the locator pins.
Another method would be to take a piece of 1/4" plate, drill the locator hole in it, place on top of spring, put rear on top of that plate. Then tack the whole thing together once centered in the car. Finish weld it off the springs so as not to input an heat to them. It's HBE (hillbilly Engineering) at best, and I've done it before, but, will get you by in a pinch. You might have to drill a holes/notch the perch to get U bolts to align with shock plate.