You might want to try using some duct tape to cover up the holes in the rust. Duct tape is a proven gas mileage enhancer, as you can learn by doing a search on it. Color doesn't seem to be to critical.
If you are getting 20/21 out that truck with an Automatic, I think your doing really well. I have a 67 Ford Pickup that get's 11mpg, uphill, downhill, highway, city, loaded, unloaded. It doesn't seem to matter. I think they just use a nozzle that squirts down the barrel of the carburator at whatever rate the mechanical fuel pump can pump gas.
I've always been fond of getting one of them dinosaur trucks from the 70s, doing Phil Knox's aeromods, and putting a nice, large V8 diesel under the hood. 35 mpg should be possible, with stump-ripping performance to go with it.
I have a 1998 Ford Ranger, extended cab, 4 cyl, 5 speed, and usually have a simple cab-height topper on the bed. An average tank of gas, used for all kinds of driving, but mostly city, over a month or more (I only us it when I have to), will give about 22 to 24 mpg. If I drive on the interstate at a reasonable speed (60 to 65mph), I will get 30 to 31 mpg. A friend with a similar truck, but no topper and no extended cab, gets similar mileage if he manages to keep it to 65mph or slower on the interstate. It how fast you go and how smoothly you drive that makes the really big difference. And of course, the engine size and transmission help.