carb options for Dodge 318

Hello,

I've got an old Dodge truck has a 318 w/ the original 2 barrel. I'm thinking of changing to a 4 barrel - would it give me any more gas mileage over a 2 barrel? If so which one?

I have a square bore intake manifold.

I was thinking of maybe one of those smaller Holley 600cfm vacuum

Or how about a spread bore intake and a QJet?

I was thinking a carb w/ small primaries might give me a tad better mileage if driven mildly and a little extra power when towing/passing. I'll also be adding a dual exhaust (no headers keep the manifolds) maybe 2" each.

thanks!