The 7.5 rear is pretty weak. It's not made of glass like some would have you think, but it is considerably weaker than the 8.5 in the TR cars.
My 99 Trans-Am cut high 1.6's low 1.7's with a six speed manual for about 2 seasons with the 7.5 ten bolt. Once I started getting the car to really work, and get into the low 1.6 territory, I started breaking ring and pinion sets.
A friend of mine has an 98 Formula that cuts 1.5's on the 7.5 ten bolt. His car is an auto though, so it loads the rear and doesn't shock it as badly as the stick does in my T/A.
With the Buick being an auto, I would think the 7.5 would be adequate as long as you weren't making any real power. If the car was making less than 450 RWTQ, I don't think you would have a problem.
None of the guts will interchange from one pumpkin to the next, but you might be able to swap the whole thing, from brake to brake. I don't know what all would be involved, but it might be worth looking into. You'd come out with the disc brakes in the back too.
If all you have is the Zexel or Torsion center section, you'll never get more than $100 for it from the F-body guys. SLP sells the factory posi unit from the F-bodies for $100, so not much of a market there for the stockers.
Good luck, let us know if you decide to undertake the complete swap!
