Looks like the top fuelers have some aerodynamic appendages, like the body running back to the engine and the angled fairings in front of th rear tires to deflect air better and the skirt around the bottom of the body.
They say most of the aerodynamic gain can be made in the back of the car, so maybe some sort of fairing or splitter in back of the rear tires.
This is probably optimized for downforce, but shows the principal:
This is interesting:
