Welcome!! :) Thinking about heads is smart. Gas mileage and performance depend on a good head. Cheapest route will be the 318 heads you already have IF they are in decent shape - meaning they don't need all new valve guides and seats. You can home port them for about 30 bucks (I have how to videos to get ya started). Cut the heads .030 (about 150 bucks). Springs can be bought between 50-70, pending on the cam you are thinking. Often you'll at least need new exhaust valves, another 50 bucks. With gaskets and valve seals, you'll around 350.
