its easy to make any engine get a reasonable mileage even with a built motor...
the motor needs to run in its rpm range first... so with a big cam hyw gears are gonna kill you... you need to be cruzing at 3 grand or so going down the road...
i get 20hwy with my built slant... 600DP holley, 246* @ .050 cam, 3500 stall converter and 3.91's... i just go 63mph and i got 20 all the way to vegas and back for MATS last year...
its all about efficiency...