@orangeman I think there are several reasons for the rot.
1. Seems all 426s of this vintage had rot here. I looked at buying a whole cab from AZ or somewhere dry and they had rot too.
2. The machine was a town owned machine so I am guessing the operator had salty boots. They probably used this machine to load the snowplow sanders with the sand/salt mix used in the winter.
3. When I got it someone had replaced the windshield with a poor fitting piece of plexiglass glass. I leaked like a sieve anytime it rained. There was like a 1/2" gap at the top where it stuck out and water just poured in. The doors did not seal when closed, adjustments were way out and weatherstrip was damaged.
It now does not leak when it rains. The rot was only on the cab. The machine's frame is excellent. I originally was just going to just deal with the floor. Once I got started I decided to just cut out all the rotted stuff and replaced it with new metal. I treated all the cavities with Waxoyl and painted everything with a rust encapsulating primer.