The 1987 Pontiac Firebird Pro-Am, regardless of whether it's a base model or an optioned-up version like the "II," did
not come standard with a Posi-traction (limited-slip) rear differential. It was an available option. Therefore, some 1987 Firebird Pro-Ams had a posi-traction rear end, and some didn't. You'd have to check the vehicle's documentation or the axle code on the vehicle identification number (VIN) to be certain.
The F-body (Firebird/Camaro) frame from that era does support a V8 engine. The Pro-Am came with V6 and V8 options, so the frame is structurally capable of handling the weight and power of a V8.
