Carb advise

[aboz]

I've got a 250 inline six from a 1970 Nova in my '63 stepside. I'm getting ready to install a Clifford water cooled intake manifold with a matching set of street headers. I'm replacing the old single barrel with a Holley 2 barrel. Is there a huge difference between the 350 cfm and the 500 cfm? At this point I care more about gas mileage than torque or horsepower. Will the 500cfm be a waste on the factory cam or just too much for my engine? Need advice quick.