neutral safety switch

I guess this is the right forum. 67 Dart converted from S6/904 to 360/727.
The trans originially had a 2 prong switch. Then the shifter selector shaft starting leaking. Due to work demand, I took it in for repair. While under there, the mechanic said the safety switch was leaking and replaced it. The PO never had the switch connected nor is the plug anywhere to be found. The starter relay conncection for the switch is ground to the battery, allowing it to start in gear. I want to connect the 3 prong switch, where will the wires connect?
My factory manual shows the 2 prong. Anyone have a schematic or words of
advice that will help?

Thanks