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

How much overlap on the cam? If it's too "spicy" a cam, you could be sending a good amount of unburnt fuel out your exhaust. Does it smell rich?

Also, do you run a fuel return line or just a feed?

That carb sounds big for a mild 318. I had a 600cfm carb on mine (truck cam, 360 intake, single 3" exhaust, pretty mild) and it was way more carb than I needed.

Just my thoughts.

-Mike