Hi, i have a 1986 firebird that originally had the 2.8L base model engine AKA the sports coupe berlinetta. It's time to start looking at the rear end and before i start ordering my LSD etc, i want to be absolutely positive on the specs. I'm also out of town away from my car so i'm hoping that there wasn't more than one option for the rears in 1986. The things I have found so far are: 10 bolt 7.5 inch 3.42 gears I'm not totally sure on the spline or anything else I may need. If someone could just guide me in the right way, that would be awesome. Thanks in advance.

Should be a 26 spline

The axle splines will be 26. GM didn't go to 28 spline until 1990. If you can find factory axles or buy aftermarket axles for a 90-92 third gen, you can install a posi for 28 splines with the new axles although your V6 won't need the larger axles.

If you go to the TGO home page, there's a huge database about all the third gens.

A 1986 SC Berlinetta was only available with a 2.8 but had the option on a manual or automatic transmission. Gear ratio offered to both was 3.42.