I have found a couple mpg is pretty easy to get with any number of tricks, getting big gains is hard even combining everything I know. A couple of the best improvements come from putting in ford yellow fuel injectors, slotting and advancing the crank sensor. Next up thin oil helps, long tube headers with a high flow exhaust. Hughes airgap intake (although this costs you noticable low end TQ)helps mpg but I would not combine it with 3.55 gears and typical 4x4 size tires or anytowing. Skinny tires. No lower than 3.55 gears, the deeper they are the worse MPG, the magnum engines deliver the best MPG at low RPM (1400-1800) and lugging them pretty good. A short duration high lift cam helps. Thin head gaskets help bump the compression up and that helps also, I like to buy one mr gasket MLS head gasket seperate the layers and use one outer layer of the gasket for each side, it is the equivelant of milling about .040 off your heads.

Certain mods are not worth it just for MPG increase but other side benifits may make it worth while any how. The SCT with the custom tune is gonna be very expensive but should help MPG a lot as well as give you more power, same thing if you pay someone to swap in the thin head gaskets or if you buy brand new injectors(I usually get a set at the JY for about $50 so it can be very cost effective)