built up 318 oddly low MPG's? hp rating?

The 318 I have in my car now gets 10-14 mpg now. Like C-130 said, you don't have enough compression to use the larger 340-360 heads. I looked around for a good set of 302's for mine but couldn't find any near me. I have a 360 on the engine stand now waiting for the heads to get done. If it was me I'd get a set of 302's or early (66-67) 273 heads and get them done. That will help some. Re Jet the carb and hook up the vacuum advance. I run a comp cam too, just about the same size as yours. You need to find a way to get the port velocity up, closed chamber heads and a intake with ports to match the heads. Is the 650 a vacuum secondary?