Provided your truck is 2wd and you putt it around and drive mellow, you can drop in a smaller engine and improve it. My 300 with a 4 bbl and efi manifolds/exhaust averages 15.4 city and thats with 3:73s and a t18 while cruising around at 45mph.
I'd imagine you could probably get similar with a 302 and the right driving habits. However my engine makes all of its power below 3000rpm and even when I drive it like I mean it I seldom ever go above that so maybe not.