Welcome,
My advice on sheet metal after having earned a living for a while at a restoration shop repairing and replacing it... If you're doing or hiring first class work done where the final product (bare metal) is as good or better than original it pretty much never pays to repair/replace significant rust. For what you spend in labor/time you can usually buy an exceptionally clean truck, cab, panel or what have you for less money, these trucks are not so rare.
If the quality of work is lower, for example a patch panel lapped or just tac welded in and bondo over, it may cost less than sound used sheet metal. Though you'd have a lower quality result that may be just as satisfactory depending on your expectations and desires.
Bruce Bugay in Glen Allen has been a great source for NOS parts and some used. I have not talked to him in a few years so not sure if he is still in the business/hobby.
https://www.stovebolt.com/ubbthreads/ubbthreads.php/users/2480.htmlGrigg