The way I understand it, it is best to have the rear jigged-up and checked for straightness after the brace is welded on. Anytime anything is welded on a rear end housing, there is a chance of it "moving".
My neighbor across the street owns a local welding/chassis shop and he has a jig made for a A-body 8 3/4". He said he will do it for $20 & a six pack!!!