Ok, the following assumes that apart from the alternator, all of your wiring is completely stock and the gas pedal start was working before you installed the alt.
Follow the oil gauge line to where it is connected to the engine. Disconnect the line and remove the existing fitting. Take the fitting to your friendly local parts store and buy a tee fitting, a short nipple, and an oil pressure warning light switch, all with the same thread. Also get a wire terminal which fits the switch. Install the nipple and tee fitting on the engine, and then install the switch and the original gauge fitting on the tee. Reconnect the oil gauge line. Use teflon tape or pipe thread sealant on all the pipe threads (but not the gauge line nut) to insure against leaks. Connect a piece of wire to the switch that is long enough to reach the stock voltage regulator. 18 gauge will be fine for this.
Disconnect either battery cable. Remove all wires from the stock regulator. Connect your alternator output wire (or the stock GEN wire, if you are using it for that purpose) to the wire(s) which were formerly connected to the regulator BAT terminal. These would be the red and white wires if everything is stock. Use a small screw and nut to make the connection and tape it afterward so it can't short against anything.
Take the light green wire which was formerly connected to the regulator GEN terminal and connect it to the wire you ran to the oil pressure switch. Tape this joint. Tape the pink wire which was formerly connected to the regulator field terminal. If you have any unconnected wires left at this point, tape those too.
Reconnect the battery cable, and you should be done.
Here is a stock 59 Buick wiring diagram. All of my comments above assume that this is what you actually have on your car.