I have done several '71 GTXs. Like the E bodies, they are very prone to rust. Once started, in my experience there is no way to really stop the rust. I even had one dipped by Redi-Strip and it wasn't rusted real bad. Still had some spots coming bad just a couple of years later. And it really surprised me that when we got the car back from Redi-Strip that a small rust spot would actually have rust for a foot or so around that spot. Stripping the car revealed that there was far more rust under the paint that wasn't even visible than there was rust that was visible.

Rust is like cancer. The only way to deal with it is to completely remove ALL of it. Which is basically impossible in a car that has a lot of visible rust.