318 Four Barrel Intake

Since 2bbl's and 4bbl's are rated at different pressure drops, or vacuum readings, what you really need to look at is the size of the barrels. If you could get both cabs on a flow bench and compare, that would help. But since we do not have these in our garages................

The 2 bbl. is good on gas. Period. Like Roy said, you can't give into temptation and floor it for more. However, while driving on the road at a moderate speed, only the front primary should be open. The size of the small 500 cfm carbs primarys are smaller than a 2bbl.'s. This should help increase fuel mileage. The velocity through the carbs smaller throttle bores will remain high and keep the air and fuel moving rapidly and well atomized.
This is where the mileage comes in.

Other improvements in the intake, exhaust. Easy in, easy out free up power. A better ignition system would also provided more mileage by getting more power. Or bang out of the air and fuel charge. A complete burn of the air/fuel charge in the cyclinder rather than most.

Things I have done in the past to improve mileage were;

Open air cleaner
K&N filters or equal
new, smaller carb
better intakes
headers
free flowing exhaust, duals
upgrade ignitions from start to finish
remove accer.'s not used on the engine including fan. Went to an electric

And tune tune tune that carb.