Looks good

If you plan to keep the drivetrain basically stock, you can upgrade the rear with OEM components with no issues. They only get weak when you start putting more serious HP to the pavement.
As for the engine, get a cold air package, a good exhaust system, and a pro tune on the computer. It will really wake her up.
Yours manual or auto?
I've had several Firebirds. Atleast one of each generation and bodystyle.
'68 Firebird 400/auto convertible (with A/C)
'75 Trans Am 400/auto
'87 Firebird 305/auto Ttop (this one a had the longest, I upgraded her to a built '89 Corvette L98, 3.23 posi, and discs on all four corners, race tuned suspension, and plenty more. I autocrossed it competitively for about 5 years before we built a dedicated racecar. It was a fun car

'89 Firebird 305/auto Ttop
'89 Trans Am 305/5spd Ttop
'95 Formula LT1/6spd Ttop
'96 Trans Am LT1/auto Ttop fully loaded -the CD changer