First of all, welcome to ecomodder!!
Looks good. You should definitely get rid of your mudflaps though. A belly pan will help A LOT on such a high vehicle as well. If you are going to block the entire grill you should watch your temps carefully to be sure that you do not overheat (I have experienced this), I think you will be ok though because you have a bunch of airflow from below the vehicle.
I remember reading somewhere that about 50% of the aerodynamic drag on larger vehicles comes from the wheels and wheel wells. How accurate that number is, I am unsure of, but you can expect to see good results by covering your wheels with smooth hubcaps and if you are willing, rear wheel skirts.
As for the aerocap (truck bed cover), for best results you should carefully consider the angle of the roofline and design something to fit the "aerodynamic template". For that, you should take a picture of the side of your car, from a distance and then follow
this thread.
What are your mpg goals? How far are you willing to go in terms of aerodynamic mods?