I know this is an old thread, but in searching i found some useful information that supplements the info provided by Tekenaar...As oldopelguy said, you need both safety devices (inertia and OP cutoff switches) for complete EFP safety. The OP cutoff switch needs to be the SPDT (single pole, double throw) type with 3 contacts (NAPA OP6610) and wired as follows: common - FP+, nc - ignition switch start contact, no - switched 12V.
http://www.studebaker-info.org/text3/fpsafetysw.html said:(June 2007)
GM oil pressure switch..... 1/8" NPT screw-in switch with three flat-blade terminals. These switches areavailable from most local auto parts stores. Three part numbers: NAPA (Echlin brand) p/n OP-6610 or Standard brand p/n PS-133 or Delco PS-9. While at the parts store, also get a standard sealed-beam headlight 3-prong socket connector. This connector fits the new oil pressure safety switch. You may also need a couple of 1/8" pipe tees and short nipples if you also have an oil pressure gauge.
The switch has three prongs marked "I", "S" and "P." When there is NO oil pressure, the "S" and "P" are connected. When there is oil pressure, the "I" and "P" are connected.
The "I" wire is connected to 12 volt ignition feed ( 12V side of the Ignition dropping resistor if you have one). The "P" wire goes up to the pump relay. The "S" wire goes to the starter solenoid "S" terminal to energize the pump during cranking.
If you want a pump priming switch, for occasions where the vehicle has been idle for a long period and the carburetor bowls are dry, a momentary bypass switch is wired between the "I" and "P" contacts. With the ignition on and the switch pressed, the pump will run to prime the carburetors, before turning the key to the start position.
the S wire from the pick-up terminal of the starter solenoid is energized only when the key is turned to start the motor. The current goes through the normally closed (at rest) contacts of the oil pressure switch to the fuel pump relay (between contact "S" and contact "P"). When the engine starts and the oil pressure rises, the switch transfers, causing current flow from the ignition switch (contact "I") through the transferred switch contacts to the fuel pump relay (contact "P"). As long as oil pressure is maintained and the ignition is on, the pump keeps running. If oil pressure is lost from an oil pump failure, the engine automatically shuts down.
With this setup, you can use any fused relay, like the ones with a mounting ear, and any suitable inertia switch.