All this talk about gearing but you're overlooking the obvious - get a bigger fuel tank!
As was said earlier, these cars never got good - or even decent - mileage compared to modern cars. Have you considered an overdrive? They're pretty expensive and it might take a long time before the fuel savings offset the cost - except here in California where an overdrive costs about the same as a couple tanks of gas
A higher (lower numerically) gear ratio might help a little, but you sacrifice performance. I just look at it like this: being able to drive a muscle car gives a sense of gratification that you just can't get any other way, so I'm willing to pay the price.