Drilling floor/rockers for seat belt mounts?

My '70 Duster originally came with a bench seat and the two-piece shoulder and lap belts. I want to install some modern retractable 3-point belts like the ones from Wesco, etc. but they all need to have another anchor point made in the side of the rocker. Is there a way to get behind the rocker sheetmetal to weld a nut, or can you just weld one on the other side sticking out into the interior?