Roof repair problems!Advice?

[old Chevy guy]

I am going to embark on a cab swap for my 72 stake body dump.The old cab is in bad shape and not worth repairing,and Ive done a few cabs already.One of which I did about 12 years or so ago for a guy had new floor rockers cab corners floor supports etc. long story short I got it back about two years ago and figured to swap it on to my truck, job done right? Wrong since then the inside of the roof above the windshield is rotting out near the visors,[wasent showing when I got it back]. Thats the same problem my old cab had [among others] and the same thing that was wrong with every other cab Ive looked at to swap before. does any body know of a repair panel avalable for this ? or has any body repaired this before,got any pictures?