There is a removable panel in the bed that allows access to the rear shocks for replacement. This often leads to water getting through and rusting the floor below (which is the area just behind seat). If you are saying it has "rusty door bottoms and rear quarters," I would bet the floor is VERY rusted.
Years ago I bought a '62 Ranchero for parts that didn't have any rust visible from above, but the floor below the access panel had rusted through almost completely side to side. My advice is to check the floor very thoroughly.