Aero mods are going to help, just gotta start throwing things on there. Take a look at the template if you haven't already:
http://ecomodder.com/forum/showthrea...ate-33673.html
Building out the body with a boat tail or something is going to take a lot of time and effort and probably money. But, as others have already mentioned, there's some low-hanging fruit in the rear wheel wells, just cover those up with coroplast. Next easiest thing would be coroplast under-body panels, or maybe building a better air dam on the front.
I don't think anyone mentioned moon disks for the wheels themselves.
Basically anything in here is what you want to do to the tires. It can be done with coroplast or with sheet metal, or often times actual pizza trays if you can get some on the cheap.