355 gear vs 390

I currently have a 70 Gto conv A/C car w/ a 72 YC 455 producing about 450hp at the motor. It has a M-20 4 speed with a 390 posi rear. The rear end was leaking and the car no longer spun both rear tires only one. I brought it to a very well spoke of shop to rebuilt the rear and he suggested I go...

I currently have a 70 Gto conv A/C car w/ a 72 YC 455 producing about 450hp at the motor. It has a M-20 4 speed with a 390 posi rear. The rear end was leaking and the car no longer spun both rear tires only one. I brought it to a very well spoke of shop to rebuilt the rear and he suggested I go...