A few posts up NHRA was referred to as mandating 4140 tubing . To be exactly correct NHRA relies on an SFI spec, just like safety belts, bell housing shields, rollbar padding, ad infinitem.
The SFI program was formerly the SEMA Chassis Builder program and the whole now giant SPECIALTY EQUIPMENT MARKETING ASSOCIATION- the guys with the second biggest trade show in 'Vegas -actually started with need for a chassis spec for slingshot dragsters. SFI divorced from SEMA some years back and, although not part of NHRA, has always been closely affliated.
Because of recent fatalities/crashes the SFI program has come under a lot of flack. Part of it deals with summarily dropping chassis comitteee members because they didn't agee with the SFI head and his close friend, a prominent midwest builder who IMHOP legislated rule changes without proper engineering diagnoses that were prone to causing catastrophic failures-and DID!
Oxweld 65, an old Linde number was the rod of choice for years but I UNDERSTAND it's no longer made- if anyone knows different, please post.
You'll be safe in TIG welding 4130N with ER70-S2 rod- commonly available- don't be mislead by chromoly rod -unless the whole assembly is undergoing normalizing after weldment.