In addition to what Zenrat said about the unlimited build, the car must appear street legal. Headlights, taillights, license plates and appear to be capable of running cross country at a high rate of speed. A pro street with a ten gallon fuel cell for instance (someone tried it) would not be a good choice.
And Zen, no one might have tried a full Nascar racer, but the Nascar chassis' have been used a few times. My Monza and a clever Nissan Cherry come to mind.