you may be able to lower the box quite a bit if you don't mind the tire humps on the floor.
the truck yards are full of old fairings so after lowering find some thing that fits.
I like my air-tabs, may get a bit better milage, but the buffeting and side wind difference is dramatic.
hang some big light weight boxes under your box, then sheet the bottom axle to axle to clean up the airflow under the truck.
There are other ideas, but you really have to run the dollars spent over potential savings.
does it make financial sense to spend 1000 dollars on a gear change for a 1/2 mpg improvement when you only drive a few thousand miles a year?
My project started out as a mpg project, I get 10 at best with ULSD and I'm at the point that the easy stuff is done and to change out anything else doesn't math out even with 4 dollar jelly.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio