Unfortunately that is one of the problems with double or multiple layers of panels hiding hard to reach cavities behind. They are all over these cars and bodies weren't dipped back then. Your concern is real, but do a good inspection of door sills, inside fender wells against the outer body panels, along the frame where panels have been overlapped and spot welded, the underbody, etc. It means taking some interior panels, sill covers, etc off. I believe Eastwood (and others) make products with a long, skinny spray nozzle to get into areas like that, if it's not too late.