Pinion angle and diff centering

That's way too complicated. Here's how I do it. I use concrete blocks, because it works for ME. Flat and level concrete floor of course. Set the perches on the blocks and get them to "whatever" width you're needing. Put the rear upside down into the perches. You'll need a third stand under the pinion nose, of course. Now, double check your perch locations are correct center to center and equal distances from either end. Lastly, position the pinion 5 degrees nose DOWN. Double check again that you didn't move on the perches center to center or side to side. Tack the perches on and double check all the measurements again. If they're good, flip the rear end over so you can get to the perches good and weld them on solid. Since you were upside down and nose down 5 degrees, now, when you right side up, you will be 5 degrees pinion up, which is right where you want to be.