In short, yes, there was an overdrive option. They do pop up quite often. You'll need the later version, an SM326 with the H12 overdrive. There's also some electricals that will need hooked up plus a cable for the lockout, otherwise it's the same as your existing 3 speed and a direct bolt in.
I have one in my '55, it's very enjoyable to drive, turns about 2000 rpm at highway speeds.
