I have a 87 firebird and I'm building a 496 8.1L
motor was wondering if I can use the original rear
end and if not what rear end would be the best to
hold up with the motor
I cannot speak to what you will need but it is most certainly going to have to be stronger than the original rear end. If it is a base Firebird, it most likely had a V-6 or maybe a small V-8. Both probably had less than 200HP. The rears on these cars would not stand up to the monster you plan to install.