Learn how to tell the difference between 7.5" and 8.5". GM didn't seem to have any set rules on what car/engine combo got what rear end or what wheel bolt pattern. I suspect a hi-po model would get the 8.5" for sure, but some of the mid-sized turbo Buicks from the '80s got 7.5" units, so who knows for sure.
Last year I junked a couple of Buicks that illustrate my point. One was a big old '82 Park Avenue that had a 307 Olds motor, a 7.5" rear end, and the 5-on-5" wheel bolt pattern. The other was a slightly smaller '84 LeSabre, also with a 307, but it had an 8.5" rear and the 5-on-4.75" bolt pattern.