It'd probably be easiest to use a limit switch like Jerry suggested down on the side cover of the transmission. One of the cover bolts would be the the spot to secure a bracket. Another option would be to use a brake light switch, but those work backwards of what you'd want.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.