I would have done it a little differently. It would take longer, but I would sleep a little better at night.

I would put the rear up in there with the pearches (not welded) and bolt it all down with the weight of the car on the suspension. Measure and adjust all angles, then tack weld. Pull rear back out and fully weld the perches on.

I would do it this way, because you are assuming a few things. If your old rear was put in and then you added different springs (SS or others) the pearches may not be the same. For instance, my SS springs require the pearches to be at different angles for the left side vs the right side. If I did it based on a stock 8 3/4, I would have welded them up both sides the same, and when bolted down, the spring would be under stress (Twisting) all the time.

Just a different perspective.

Love your car Unlawful! Good Luck.