86 firebird rear end

On the car im going to look at was originally a 5.0 V8 car swapped to a 350

is the stock rear end a 7.5" or 8.5?

7.5" it's a good rear end.

If you have access to the car itself you can check the RPO sticker under the armrest (check the vin to make sure it's the one for that car.

Some years have a second sticker under the carpet in the hatch area I've heard.

And some years you may get luck and the axle code is on the rear assembly.