Trouble there is it has to be chrome moly and Tig welded. I had my Daytona (mild steel) updated to 7.50 back in 2008. My 74 duster is a 6.50 NHRA cert. chrome moly is crazy high in price now.
True, but the SCTA doesn't have any specs and the burden of proof is on the builder. If you design the cage to SFI 25.3 but substitute chromoly for 1-5/8 .120 DOM you might be able to convince tech it's a sufficiently safe design.