right now my car has some 3.23 non posi gears in it. If i put in some richmond 3.73's how much would it shave of the 1/4 mile? Also what rear-end do i order them for? is it the 7.5 gm 10 bolt? How do i tell if my car has the stock rear end, cause i think it may have been changed...

 

Your 91 has a 7 5/8" 10 bolt but it's still usually called a 7 1/2" since the parts are interchangable.

What makes you think the diff has been changed? RPO codes in the glove box or axle codes on the axle tubes don't mean much since internal parts can be changed. The only way to really know what's in the diff is to pull the cover off and count the teeth on the ring and pinion. Pull the axles and count the splines. You should have 28 splines. The older 10 bolts have 26 splines.

As for gear selection it will depend on how the car runs. More gear will increase the torque multiplication to the tires. For 1/4 mile racing you want the gear ratio and tire size matched so that you cross the finish line at red line in high gear 1:1, not OD. That will give you the best performance.