[elhigh]

For starters, those rear wheel openings are gigantic and for the most part have no wheel in them. Even partial fender skirts would do you a world of good up there. If you're never loaded enough to fully compress the suspension on your worst road, then you can really cover those up and not suffer at all. You could probably add partial skirts to the front wheels, too.

You don't have the big HD aluminum wheels with their projecting hubs, so you should be able to find some smoothie wheel covers, for the rear wheels at least. You may want to consider leaving the fronts uncovered for thorough brake cooling. There's an awful lot of room under the truck though, so that may not be necessary. Wheels are great big air churns; at 60mph it takes about 1hp just to turn the wheel against its own air resistance. Reductions there are reductions in the fuel bill.

I bet those openings in the bumper are optional. They're handy for stepping into when working under the hood, but they aren't big enough to add much for cooling. If you're going to start blocking openings, that's a fast, easy place to start.

Pull the mirrors in on the freeway if you aren't towing. The right-hand one for sure.

The step rails are really out there in the wind. Clambering up into a 350 without one is a chore, so you probably don't want to pull it off and leave it off. A longer, contoured model that fairs more cleanly into the rocker panel would probably present less drag.

An aftermarket air dam could reduce how much breeze gets under your truck. BONUS: reducing air pressure under the truck improves the effectiveness of hot air getting out of the engine bay, thus offsetting increased cooling load of blocked-off grille openings.

A smoothie bed cap like Bondo is making. It would look awesome on your ride and keep it fully functional.

Lowering the truck would reduce under-body drag.

A belly pan would reduce under-body drag.

Most of the aftermarket diesel chip manufacturers say their product both increases power and fuel economy - that's worth checking into.