Back before the kickdown switches became available again, I used a MicroSwitch that was available through any industrial electrical supplier with normally-open and normally-closed contacts. The one I was using had a spring arm and a roller that could be bracketed to contact the throttle linkage at the proper throttle opening. The same switch could be used as a combination neutral safety/backup light switch for custom automatic transmission installations.
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!