[mr48chev]

Best I can say is to cut the bad places out while saving them for patterns and clean up around where you cut them out so you have no rust left to deal with.

Then make your patch panels or have them made as I suggested earlier or find a cab with those panels that you can salvage. Maybe someone has a cab that isn't rusted in those areas but is otherwise scrap that can serve as a donor. I still think it is possible to make the panels and trim and fit them to the cab and (then) find someone local who can weld them in place for you. I built my first T bucket by carrying the parts down the street to my friend's house to have him weld them up or carrying them to the welding shop in town or the body shop in town to have them welded.

I don't recommend buying one of those cheap Harbor freight welders but after a bit of practice you could be able to tack panels in place where you wanted them and tack pieces together the way you wanted them so you could take them to someone who could weld them together for you or if you have the right hookup in your shop get someone to bring their mig over and do the finish welding at your place. One does what one needs to do when you are building these trucks and don't have the skillls or equipment to do the job.