No idea where to start

Hi, new to the forum recently bought a 67 Dart four door with a slant six that runs (sort of) and discovered after pulling the disgusting carpet out that there is some pretty serious rust on the passenger side front floorboard. Strange because there is no rust anywhere else aside from minor body rust on the exterior anyway not really sure how to fix this. It's bad, two large holes with spots that could also give way to large holes. Any advice would be appreciated, also if anyone knows of a reasonably priced upholsterer in Phoenix let me know as I have two bench seats in need of new everything!