Unfortunately, unless you get lucky and score an 8-3/4" rear, you will get an axle that requires a good bit of cash to set up gears. The 8.25" rear is a little easier to find, and generally cheaper, but you won't find one stock with much lower than 3.2 gears. Other than strength, you won't gain anything. As another poster mentioned, going low in gears will affect driveability. When I cobbed in the International rear with 4.27's it knocked my gas mileage down a bunch. The performance was so much better, I didn't care. If you switch gears, I wouldn't go higher than 3.91's. 4.30's would be my choice.
The 5.0 class is tricky. I could beat 5.0 Camaros easily, but a 5.0 Mustang is a different deal, much faster. With the shaved head, home porting, 2 bbl, and the low gears in a 2,700 pound car, it was still maybe a 15 second ride, or a little faster. Slants cost money to go fast. I won't try to talk you out of it, hey, I did it, twice in fact. If you are on a real budget, and speed is most important, a junkyard 360 and trans will save you a bunch of money.
