318 drinking gas like hell

auto trans? = bad mileage if it's the old style converters
360 heads on 318? = bad mileage because of loss in compression, you have to give more gas to compensate and boy do they flow better
headers are better for mileage than stock manifolds.
dual exhaust is better for mileage than single
thermoquad carb is better for mileage than most of the others.

------ summary -----
4-speed, thermoquad, headers, dual exhaust and the right compression ratio (9:1 or higher) will get you good mileage.