Why did you start with such a small engine?
I have a 2001 2500HD 8.1L 496 Big Block, Allison Auto, 4x4. Internet lore states it can't get over 10-11 MPG. Just doing basic "nice" driving techniques with a flat tonneau bedcover will get me up to 14MPG highway. The stick shift will enable you to do more advanced eco driving, i.e. coasting down hills and to stops.
I would focus on doing your aero bedcover first, as that is probably the biggest single aero improvement you could make, and you will notice the improvement more than the smaller aero pieces that make a smaller impact. Focus on making the aero shell
useful, i.e. hinge or other mechanism to open, water tight storage when shut, and not just a bother that needs removed to do "real work". Since I infer you know how to weld, I would suggest you use that skill to make a really nice one. Think about things like a built in toolbox accessed from the driver side, but that doesn't interfere with loading things the full width between the wheel wells. Or even get a hatchback or two from a junkyard and put that together into a shell, using the factory seals and flanges to ensure it is watertight.