Don't get your hopes up about getting mid 20's in a square body, it probably won't happen. I am getting 17mpg in the city, which is where I do most of my driving, and only slightly better on the highway. My truck is a '71 short bed, 2004 5.3/4l60e with 3:08 gears. I feel like the gear hurts me some in town, but should help on the highway, but on my longest trip I've made in it, 1200 miles of interstate in one day, I only averaged about 19-20, with the cruise set, although I was doing 77ish MPH for a lot of it.
