That's what I thought about doing and that is why I set this cab on the chassis for mock up. It has a lot less rust in the front fender attachment area, but both cowl sides have creases from the doors and inner and outer rear cab corners are rotten and there are a several other dents.
I have to replaces the same areas on my other cab and it has fare less dents and dings and a good roof. The only thing really bad about my original cab is it looks like someone hit the voltage regulator with a 10 pound sledge and wrinkled the top corner of the firewall.
So I plan to cut a section out of the caved in cab and fix that.