Just remember that any of the tubular framed cars are zero percent close to an original monocoque chassis. Thant includes the RCR monocoque and CAV monocoque which has similarities but far from being true.
But they do offer a wider variety of options of what you want to modify and adapt for your hearts content options.
The purer you want to be, would be with Holman Moody, GM Automotive Engineering,( Gelscoe), Safir, Superformance, if you can magically find ERA ones or if you like to clean up galvanized metal prior to welding GOX if you can even make contact.