SteveG75

You could go with the 3.70 gears and if the highway rpm gets too much for you look into doing a TKO swap to get either a .82 or .64 OD. If this is an original 435 car, I would recommend a TKO over a T56 just to avoid tunnel/crossmember modifications.

62corvette

FWIW, Standard rear on an LS6 in 71 (only 425 hp) with M22 was 3.36.

Sunstroked

I have the 3:55 in my differential, wide ratio 4 spd. Engine was built to produce 435 hp, never dyno'd it. Its a good gear for what Im doing. The wide ratio Muncie has a lower 1st gear then the close ratio. I had a 4:10 in a Camaro behind a built small block with a Turbo 400. That was a good gear for what I was doing with it.