[AndrewJ]

I think that the thingie in the bed will work out if you can get the 13º angle that you're supposed to have. To measure it, take a profile shot of the truck from as far away as you can. Then measure with photoshop or a protractor.

As for underneath. More is better. The little blue triangles ain't much, as long as you're under there just cover up as much as you can. You'll have better results that way.

Also think about making some sort of wheel spoilers. I know 31" is a lot of tire to hide from the airstream, but you can at least make a dent in it.

Edit: Oh, and look up aeroheads posts, he mentions that the best bed cap is only a half cap. I don't know if he elaborated as to what half (tailgate or cab half) is best to cover, but it would still leave a lot of usable cargo space. Especially if you made it hinged somehow....