63 C20 stock I6 which rearend should i look for

[defgone]

nearing the process of swapping out front cross member and rear end....i am leaving the stock I6 230 (will add offy intake, holley 4 barrel, dual exhaust manifold)...my questions are:

what gearing is my stock 1963 C20 rear end?
what gearing should i look for to be able to go a little faster but maintain decent gas mileage??(trying to be able to get around town a little quicker and occasional cruise at 65-70mph)

would a 308 rear from a mid 70's c10 be good??