From watching our derby races last weekend, the shape of the wood seemed to have very little to do with it.
The winner in my division had a flat body of wood, that was only about 3/8 of an inch think, and he had more weight in lead than wood.
You gotta have the axles straight as an arrow to cut down on friction when the inside of the wheels rub against the track. Lube the crap outta the axles, polish the axles to a mirror/smooth finish, and keep the weight even across the car, with maybe a touch more weight towards the back.
As big as my truck looks, it never lost by more than an inch.