Getting your front back down to stock height and lowering the rear to match is the way to go. Then add the air dam, it should make a noticeable improvement. Trucks are wide open for aero improvements.
Seconded for throwing a sheet of plywood on the bed for quickie tonneau tests, and I recommend a rigid tonneau instead of a soft one, if that's the route you go. I think any flapping the top does is going to induce turbulence. Turbulence Is Bad.
This is someone's Master's Degree thesis digging into the various things you can add to a pickup to reduce its aero drag:
http://csus-dspace.calstate.edu/bits...d-finished.pdf By his experiments the tonneau was only good for about 1.5% improvement, but that was with a generic "truck" shape. Your real world results, as they say, may vary.
Other trucks, especially ones modded for top speed runs at Bonneville, have run with half-tonneaus covering the bed from about the midline of the rear wheels back, with a solid bulkhead at the front. This leaves a large open space between the bulkhead and the rear of the cab. I have no idea whether and/or how big an improvement that would provide, but it's interesting to think about. If nothing else it would allow you to continue to use the truck as a truck for things that aren't too big or heavy, just toss them in over the side.
I say for the price of a sheet of plywood and a few minutes with some ratchet straps, a stop watch and a few runs down a hill, what more is there to lose? The improvements could be significant.