A 3.73 would be a good choice for a hot street car.
However, I wouldn't stick much money into the weak 7.5 rear thats hung out back of that 81 Bird because you will probley be replacing it once or twice. With anything over 300hp you will snap that thing like a toothpic. You are pretty much stuck with this rear unless you want to pay the $1000+ price tag for a swap-out 9 inch or 12 bolt for companies like Moser.
What complicates things is the rear torque arm that connects the trans to the rear. These were only used in the 80s F-bodys and the 76-80 Monza which makes it impossible to swap a stronger into these cars using the stock suspension setup.
This was something that I would have had to have dealed with my 76 Monza. However, I decided to elimate the torque arm setup by tubbing the car and running coil-overs and latter bars.
The best thing you could do to make that 7.5 live is to run street tires to limit stress on the weak internals. If you run any type of sticky tire and hook, you will surely brake that thing. Also, if you run it at the track long enough, even on street tires, you will end up braking it.
-79 Malibu (Soon to have a fog fed 454 Big Block)
-Alston Backhalfed 76 Monza
w/1.48 60 ft (Motor Only)
w/1.40 60 ft (with nitrous, turning it off 3/4 way down the track) Should have high 9s with full nitrous run!
[This message has been edited by Chris Hill (edited 12-28-2000).]