The toggle-switch seems to be the easiest method.

There was a back-up switch made for SM420 transmissions (available in 1967) and there was/is a back-up switch that could be used on the shifter-pivot inside the engine compartment for 54/55 Hydra-Matic transmissions.

Supposedly earlier SM420s could be adapted to use the 1967 switch; but, the one for the H-M was/is a bolt-on. Sorry, I do not know part numbers for either switch.

http://67-72chevytrucks.com/vboard/showthread.php?t=181306
http://www.advanceadapters.com/tech-vault/gm-sm420/
http://www.jeepshots.com/shots/projects/sm420/
http://talk.classicparts.com/showthread.php?t=7345