The way I saw it done was complicated but it worked. The factory installed the exhaust before the rear axle and springs were installed but with the engine and transmission in place. What was done to replicate this was the aftermarket exhaust was "fitted" with the rear axle in place for clearance purposes, marked with chalk and felt tip pen, and then removed, along with the axle. The exhaust pipes and mufflers were trimmed then welded to make two complete subassemblies, then installed. The rear axle was then reinstalled. It took a day and a half due to some fitting problems of the forward pipes to the muffler. Fortunately all the pipes were too long, a lot better than the other way.
Jim