Re: installing 3.73
Y81 is the code for a base Firebird merchandising package. Has nothing to do with a limited slip diff.
If you do not have a limited slip ("posi") differential, you will probably experience 1-wheel burnouts as soon as the extra torque from the higher numerical gears causes one wheel to break loose. The car will sit there smoking one tire, and going nowhere. Liited slip locks those wheels together to limit 1-wheel burnouts.
The best thing you can do is buy a used rear axle assembly from a 98-02 V8 model, which came standard with a limited slip carrier. Then swap the gears.
94 Formula A3+1: 381ci all-forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams custom solid roller / TH400+GearVendors OD/4.11's / Strange 12-bolt / 300-shot N2O / Spohn Suspension / 6-pt roll bar with integrated SFC's / full AutoMeter instrumentation / MoTeC M48 Pro engine management system / a few other odds 'n ends. 800HP/800LB-FT at the flywheel.