Only one way to do it. Cut it all out, and weld in new metal. You will probably have to fabricate new sections yourself as you probably cannot pick them up from Ford anymore. As an alternative, cut sections out of a rust free (does such a thing exist?) example, and weld them in.
Only thing is, welding does change the characteristics of the metal, and unless you can get behind the repair to inhibit and seal it, the rust will eventually reappear.