I ran street gears in a 12 bolt in my Vega, with a 331, it went from 12.40s street trim, to 11.90s with slicks, (3.73s, 4.10s, 4.56s, 4.88s, all used gears), then I swapped in a 427 I built, (back to the same 4.10s), went 11.50s street trim, 9.70s on slicks, uncapped and some spray. 1000s of runs, never broke it. I put over 150 K on it with the 327 over about 14 years time, and about 20 k on it with the 427. Sold the car, it got parted out, the rear end came back to me in a 68 Camaro with an axle tube twisted loose, I fixed it, backhalfed the car, with 14x32, tuned the 454 with a 150 shot, it FINALLY broke a spider gear, then I put a spool in it...same gear....