I followed post #1 about a year ago to create a harness for a LS cruise control in my 72. I didn't want to replace the brake switch for a LS style one so I came up with a solution to use a relay and 10k ohm resistor to emulate the behavior and make the cruise module think I was using the correct gear. The brake relay is normally open sending power to the clutch input and also provides the brake a source to ground via the 10k resistor. When the brake pedal is pushed it kills the clutch signal and sends 12v to the brake input. Viola!
Jump forward to now I finally got myself a Rostra cruise control switch and when I was probing the function I realized that the set button is normally closed which does not work for the LS cruise.. doh! Another relay to the rescue to just swap the way in which the signal works. There is probably a better way to do it, but this works just fine for me in practical application. The 'set' relay is energized when the cruise is switched on and clicks off briefly when you hit set.
Since I had to relook at this I cleaned up my schematic and I'll post that here in case in helps anyone. It's a little odd, but it works for me. The diagram shows 12v coming from the battery directly for simplicity, you would want to use a switched source as I have done.
https://photos.app.goo.gl/UeRdo12Smss4MAFf7