[Widetracker]

Sonic, I'm a muscle car guy too. I looked into your motor for my wife's 65 Mustang so that I could gain efi and overdrive, but never made the jump. I could be a smart alec and tell you to get an ls1, but I won't. In all seriousness though my 99 Trans Am gets upper high 20s in mpg and still can turn 13 flat in the quarter. Just food for thought if you get a newer muscle car in the future. The new Mustang sadly is only pulling low 20s in any form. A shame for such a nice looking car.

Anyway here our some things to try for small gains:

full roller rockers- the late 5.0s have roller tips, but not full rollers
headers
harder compound tires (you'll lose a little in the quarter)
aluminum driveshaft- inland empire has them
look into some of the Cobra body components- some may increase the aero package
fiberglass or carbon fiber hood- also trunklid
drop the car an inch or two

You don't mention if it is a stick or an auto. I'm guessing auto with 18mpg. Obviously that package is holding you back. I've read claims of 28mpg with the stick version of your package.

Don't forget weight. Especially rotational mass. Lowering weight where it makes sense can improve both your performance and economy. Gearing is the most important piece though.