Thank you for clearing up 69_Malibu's post. That's makes sense. Yes, I thought he meant the oil sender switch.
That is a good point. If you wanted this safety switch, I would connect the ground of the relay #85 to one side of the pressure sensor switch, the other side of the switch goes to a convenient ground. That way when the oil pressure drops, the relay loses it's ground, and turns off, and of course turns off the fuel pump.
12 ga. should do. (10 ga. if your kinda of safety freak like yours truly, but you might have trouble finding 10 ga. in that length from an auto parts store.) So run 12 ga. to the #30 on the relay from the battery with a fuse on it, and 12 ga. from #87 on the relay all the way back to the fuel pump. Make sure that fuel pump has a GOOD sheetmetal ground. I wouldn't recommend the frame itself.