While the half tonneau should save you an mpg, you might think about putting together a plywood aero cap, as bobo333's 1977 Ford F100 sports. Shouldn't cost too much money and will probably get you two or even three mpg. I know, not much, but with a baseline in the low 20's it's going to be really tough to get much improvement.
What's your goal, fuel economy wise? Just minor improvements that look stock, or making it as efficient as a midsize car?
btw
http://www.fueleconomy.gov/feg/Power...WD&srchtyp=ymm