The Stovebolt Site Forums The Shop Area Driveline SM420 Backup Light Switch

It was recommended that RAuto part NS18 (NOpen)was the correct ball switch for the job and it will work but you don't get to put in the original 1967 location. It is possible to do this job right/stock if you locate the proper switch a (NC)one would do the trick however RAuto does not carry them so the price is 3 times as much.
The theory behind having the shift rod pressing on the switch 99.9% of the time doesn't sit well with me anyway. So to me it makes more sense and it's cheep! $8.10 NS18 switch
Pics have location measurements. Drill to 1/2" and tap to 9/16 x 18
Once the cover is off the trans it's a 30 min job. Hope the pictures help. Ken
https://goo.gl/photos/siq78UnJfcNa6Myd8
https://goo.gl/photos/ekfj6JMpLWdJx8Ms8
https://goo.gl/photos/A5YmkVhjiyrPc8a1A
https://goo.gl/photos/c83UxSskhXVNmUtC6
https://goo.gl/photos/tXSfN82ZQZBpAFa36
https://goo.gl/photos/xbT9soxrTxu9Wo5h9
https://goo.gl/photos/iUhdYraCTNrNMx4D6

Good to know as I plan on putting an sm420 back in the 59 GMC. Will the floor interfere with the installed switch in your application? Is that 1.023 inch(?) measurement on cutting the bevel from corner to seat the switch flush?

42
That's with the caliper resting on the flat. I checked the clearance and it is close but workable. However my truck has a modified auto trans floor panel.
The housing is 0.260 thick at this point and you will need to grind about 1/16 to get the switch to adjust down for correct contact.

https://photos.app.goo.gl/t45DGLgedn1Qm1eH3