Well it looks like your floors are pretty good, usually you see floors eaten out from a leaky windshield seal trapping water under the rubber mat, but it is for sure your rust came from salty roads kicking up into the back of the front fender cab area.
When you get it cleaned all out of there, it might be a bigger repair than you think after you cut it out and add the cancer margins to good clean metal. Keep us informed with pictures as you go.
Steve


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration