I don't have a diagram in front of me but lots of GM's used the oil pressure switch as a back up if the fuel pump relay failed. They used a 3 terminal switch, one for the gauge or light and the other 2 for the pump, when the pressure was to low it open the connection to the pump. If you are going to use it on a engine with a carb you would not have to use a relay just keep in mind that you have to crank it long enough to get oil pressure before the pump will turn on. One of the first that I can think of to use the system was the Vega, there were others and if you looked a wiring diagrams for early 80's GM you should find what you need.
