narrowing an 8&3/4" rear

I have a buddy that does them all the time. he has a nice jig with a 1 1/4" stainless bar and machined pucks. It is important to get them straight and square but i have yet to see a factory housing that was dead nuts. i have seen virgin housings that were off by over 3/8" from the factory. I just spent 1400 on a fabricated sheet metal housing from moser that was 1/8" off. I would never know unless i threw a jig on it.