Here is why I decided to replace the floor entirely. This was rust in between the rear seat brace (convertible only) and the floor sandwiched in between the brace and lower leaf spring mounting pad. By the time I would have removed the brace, and replaced the rear floor area, it would have looked like hell. I am going for no seams in any of my welding so it was better to start with new sheet metal. I also found swiss cheese underneath a plate someone previously added to the passenger side floor.
