When did they switch over to the newer style alternator? Back in the day I had a '71 Duster and it had the newer style alt & reg. Car was built in May of '71, so the change may have taken place sometime during that model year?
1970 is transition year. As need increased in the engine bay the ignition switch moved to steering column and the wires to and from it got larger. Those still had failures like the harness connector below the column melting. And of course any manually operated switch is subject to eventual failure no matter where its located.