It was a federal mandate to help discourage auto theft. The car had to be Park [auto] or reverse [manual] in order to remove the key from the ignition.
The parts were fairly specific-Muncie stuff will not work on a Saginaw. In fact a Muncie SB used a different lower lock rod than a BB.
If you can't find the correct lower rod [3955767, same as used on a standard 3-speed floor shift] it should be fairly easy to fab one out of mild steel rod.