I don't think they used any galvanizing until the mid 80s. My "81 245 just turned to mulch from the waist down after ten years in Maine. Its replacement, a "93 240 Classic Wagon, seems to be standing up much better. BTW, I have found many extensive perfectly solid areas with the original painting still intact under that asphalt undercoating after 40+ years. Unless you are doing a total, from bare metal up, restoration, you can perfectly well dig around in the suspicious areas till you run into clean metal and repair and treat with POR15, ZeroRust, or the like, from there back. Sometimes you can find bad spots by running your fingernails over the undercoating and listening for a change in sound -- hollow-like.
Bob S.