Hey Chip,

I'm going through the same thing with my 49 1/2 ton. I was puzzled why I had so much rust on the passenger side of the floorboard while the rest was relatively rust free. Finally found the heater was steadily leaking a small amount of coolent down the passenger side firewall to the floorboard. Won't do me much good to replace the sheet metal without fixing the heater leak as well. Just thought I'd mention it in case you have the same problem.
Dave