Are there any limits as to what you can do to the truck when rcing it ?
Remove the mud flaps
You could put on some side skirts between the wheels - maybe even making them as wide as the wheels are and closed on the bottom.
Along the same lines you could extend the skirt aft of the rear tyres as a boattail fairing.
An air dam under the front bumper.
Air deflectors in front of the tires - front only, if you do the wide skirt mod.
Tape the gap between the cab and the bed when you go racing.
Make everything as low as you dare for racing.