Rust under the rubber is rather common and is generally only repairable by replacing a large area. Although I have been replacing panels for 50 years, I would not be inclined to repair it. There are too many alignment problems that can occur. I would look for another cab. A 48 to 53 cab from a 1/2 ton up to a 2 ton will fit and there are a lot of them around. Many 2 tons get crushed because there is no interest in restoring them.
