The simplest method of doing the oil pressure sensing pump switch is to use an idiot light switch for a Chevy Vega 4 cylinder. It has 3 terminals- - - -one to provide a ground for the oil pressure light, and a "normally open" set of contacts that close as soon as the switch sees something like 7 PSI of oil pressure. If you want to, wire up a relay to run the pump, but the switch contacts are plenty capable of operating the pump without the relay. If your truck sits for long enough that the carburetor runs dry you can also add a push button priming switch to fill the carburetor bowl before the first startup, so you don't have to crank the engine long enough to develop oil pressure before the fuel pump kicks in.
Jerry
