[Christ]

If you don't already have an OP gauge, you can temporarily Tee off the pressure switch on the back of the block with a steel fitting (don't use brass, it work hardens, then will leak oil), and that will give you another port to screw in the fitting for the OP gauge.

Else you can just put the sensor in the original hole, and leave the old pressure switch out. You may or may not have to put a jumper across the pins of the old sensor, though, to keep the light from coming on.

For your sake, I hope it's just something stupid, like a small break in the circuit, or a bad/going bad sensor. It's not hard to change the oil pump, just a pain in the ass.