I did a gear swap 15 years ago on a almost-new '87 LX 5.0 I had bought. I had never done a gear swap but figured I'd try it myself.
Once I had everything out of the old rear, I compared the old Ring & Pinion to the new stuff. I figured that if I measured each of them from the same points and found them to be close, I'd just put it back in with the same shims and look at the mesh pattern by putting some white lithium grease on the gears & spinning them.
Long story short, the new FMS gears were within a few thous. of the stuff I had just take out. They were so close in fact that I just did the swap with the original shims. I had the car until 1998 and never had any problem with the rear. Maybe I was just lucky, but once we did mine, we did a bunch of other guy's cars we knew back then & none of them ever had any problems either.