will this combo kill my MPG?

I have been reading how air gap type manifolds kill your fuel mileage
so now I am stuck, I would like better power AND MPG (greedy I know)
current engine is a 318-3 2bbl low comp truck motor
I also have a 1967 LA318 (factory rated 9.2 comp with closed chamber 273 heads) I was planing on installing a pair of MAGNUM heads (junk yard take off) with a crosswind intake (Chinese air gap copy) Eddy 600 carb and Comp 268H cam (454 lift 218 @ 50 duration)
my other option is
1: rebuild the original 273 heads
2: get a pair of junk yard 302 heads
and use my LD4B intake
or just keep my trusty low comp 2 bbl truck motor

power is good, but fuel economy is better
(at least with the price of gas today)
the car is a 66 barracuda, 4 speed, 2:94 rear, 14" tires,
not interested in going fast, just cruise around for fun