I drew the outline I wanted on the side and they cut it out (make sure you tell them you want the stuff INSIDE the line, though... I wasted one block of wood on an idiot.
Remember that for lighter cars, profile counts. I'm not sure if they can still do this, but I hollowed out part of the top and melted lead sinkers to reach the maximum weight, then put wood putty over the hole to restore the shape. I suspect that a lighter car would do better anyway, since friction is a product of force.
After that it's sandpaper, bay-bee.
One final tip: put the nails for the wheels point-first into a drill, then wrap the nail just under the head with fine sandpaper or emory cloth and run the drill. This will wear away any burrs on the nail that will slow the car down.
I still have my car somewhere. :-)