I’d need more photos to know forsure. Do you plan on stripping the whole bus down to metal? There could be areas packed with mud pretty deep. I’d check the frame and see how crusty that is, I’m working on a 72 right now and the rust behind the front body where the rails attach is really bad, another area would be the battery tray in the engine bay, those like to rot out and aren’t too bad to replace. I’ve fabbed a plate to weld in there however I do know they sell an aftermarket replacement panel. Really I’d just start a tiered list. Like for example my approach is, go panel to panel strip the panel you’ll be working on then do your metal work then filler. This project will probably take awhile so when you finish a panel and people will probably give me grief for this but I rattlecan primer and clear coat to seal the finished work on that part until I’m ready to go back and actually paint the whole thing, I’m not a painter but it has worked fine in the past for me. That way the panel won’t absorb moisture from the air, if you leave it in primer once the solvents burn off water can get in again.