i dont know about efi but my carbed 300 in my 84 gets about 12 to 13 mpg and its 4x2 with 4.11 gears

 

I'd stay with the 300. any engine from 65 to 96 will bolt up. so all you need to do is find a good engine and swap over any parts that are needed. if power is what you want check out cliffordperofmance.com , jegs and summit racing for performance parts. you can also get a custom ground cam, lifter and rockers through comp cams. you will be pushing some horsepower and better torque numbers than a v8 and still have the strength of Fords greatest straight 6.