I left the rearend mounted on one side and did the other side then vice versa.
I cut the perch off then installed the new perch and tightened it down with the U-bolts after of course I had already installed the offset shackles and hangers. I am going to recheck everything before the tack welds with the car's weight sitting on the leaf springs. Find a point on the frame that is in the same location on each side and measure to that location and get the same measurement on each side. Check pinion angle also before tack weld. Some people measure between the front hubs and find the center that way but I prefer measuring from a frame location.