Replacing rear gears...go with 3.08 or 3.42?

[Crazy-J]

Hi,

I am going to replace my rear gears as they have chipped teeth. My '72 currently has 3.73's in it (did these come with 3.73's originally?). I am looking to try and improve fuel milage as well as give the engine some room to run...it accelerates nice, chirps second no problem, but completely runs out of steam on the freeway.

Would I be better going with a 3.08 or a 3.42? I am not sure how much power the engine is making (350, mild cam, performer rpm intake, performer 600cfm carb, velocity stack, long tube headers/x-pipe/2.5" dual exhaust...that's all I know of as it was built by the previous owner).

Running a 700R4 transmission.

Thanks