Keep in mind, you can only narrow so far before the springs and bearing housings will want to occupy the same space. And it works best in conjunction with the spring locator kits with offset mounts. Also, it is better to go a little too-narrow, and install long studs.Then, if you have to, you can install spacers, to center the wheels. Not narrow enough can lead to fitment issues on the outboard side. Plus you can pound the inner walls pretty good with a big hammer to "adjust" for minor fitment issues there.
What I did was to carefully measure for the maximum size. Then I bought the wheels and tires I wanted to run.(Actually I borrowed the tires from my boss who owned a major tire-shop in the area; and the wheels I had already purchased years earlier). Then I yanked the entire rear end out, moved the springs and dropped the car down over the wheels. I centered them up, allowing for body sway. I plumbed and squared everything, then measured the hub to hub distance. From the measurement I subtracted 1/4 inch per side to allow for error. Finally, I pulled out one tire assembly, and reintroduced the rear end to the still-in-there wheel. I slid the rear over to it, supported the pinion and bolted the wheel on. Then re-centered it. I was looking to make sure it was physically possible to narrow my housing and still be able to secure it on the springs, without bearing housing to spring, clearance issues. In my case it appeared that I would have to pull the springs inboard 1/4 inch per side to make it happen. And I deemed that to be acceptable. So Off my housing and axles went. And on the return; Success! with 1/8 inch spacers..for ....................325/50-15s......on 10" rims with a 4.5 backspace.