IIRC, Monte said that all of the top cars, except the current(?) Murder Nova, are full chassis cars w/ comparable componentry. Unless I misunderstood his explanation, there are more "Pro Mods" than just Chief's and Lutz's, but they simply don't look like it on the outside. I believe he mentioned Doc's & Monza's cars specifically in this regard, despite how those two keep talking up how they're racing against Pro Mods.