It's sorta easy. Start with a 55-74 North American assembly line produced car or Ranchero, El Camino etc. Car must "appear" stock. Almost anything goes (except nitrous) if you can stuff it into a factory available block that looks like one that was available in the vehicle you are using, must use original casting # heads, intake manifold, and a correct or correct appearing carb. Oh yeah, gotta have a stock body, paint and interior and no lightweight or fiberglass panels that would not have been assembly line installed. Blow it out through ported or extrudhoned original exhaust manifolds and a 2 1/2" mandrel bent exhaust system. Largest tires allowed are one # larger than came on the car, or G70's if the car came with a smaller size.
That's most of it, but check
here for more complete information.