My truck would be getting the very low end of the mpg scale for the 6.2, it's not built for that. It weighs 6300 lbs, geared too low (for mpg) and has the aerodynamics of a barn.
Canmore gets 23+ mpg on the freeway with his 83 C10 pick-up which is damn impressive in my eyes.
6.2, 700R-4 (or NV4500), low lift, 32's, and 3.42 gears would get you solidly into the low 20's.
Rene