The early SM-420's had a bracket that bolted to the top of the transmission case for a backup light switch, and a tab that attached to the shift handle with a setscrew to operate the switch. It's fairly simple to fabricate a similar switch setup if the original isn't available using a push-button starter switch and some angle iron, a small hose clamp, and other standard hardware.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!