But joint on roll cage?

I could be wrong but I believe it's legal if done correctly. My uncle had an 7.60 sec front engine dragster several years ago with a frame rail modified in much the same way- chrome moly frame rail butt welded with TIG with a sleeve and rosette welds on both sides and certed to 7.50. I believe the rulebook allows this in order to allow for evolving rules for older chassis. Again, this was a few years ago though, so I'm not sure about current rules. Anyone have a current rule book with official language?