Compu cams XE268H question

So I've got the 268 in my 72 Dart Custom 318. The motor was completely rebuilt and put in by the previous owner and has less than 500 miles on it. I was told its .030" over with flat top pistons(he couldn't remember the compression ratio), rebuilt J heads (couldn't remember valve size), the aforementioned Comp268H, and then was topped off with the stock 318 exhaust manifolds and stock 2bbl setup. It has true duals and has been converted to electronic ignition too. Stock a904 trans and stall, and stock open 7.25" rear with 2.73 fd. The car runs fine and sounds great, but gets really lame gas mileage for a 318! It's only getting 9-12mpgs in town and driving gingerly.

Now I've been told that that I need to put on a 600cfm 4bbl and decent intake ASAP and that it'd help economy and power. And I was also told that the problem was that the cam was way to much and that is what is killing the economy.

I've got a Eddy AirGap with an Eddy 600cfm carb laying in the shop that I intend on throwing on, but will it help economy or is the cam the culprit?