I've seen a lot posted that could apply. How big a race car? More than one?
I'd look through
Buy a Trailer (instead of a pickup truck) or
Improving a box cargo trailer . There's a lot on teardrops and Hi-Lows, but you might also find a lot of food for thought in the Tractor-trailer wind tunnel work.
If I don't find it someone will post the white aluminum car hauler with the split open boattail.