From Doug Meyers:
You need a oil pump priming shaft. This shaft is inserted into the oil pump drive in place of the distributor drive. You hook the other end to a drill and turn it in the direction of normal engine rotation until you see oil pressure on your gauge. Then you reinstall your distributor drive gear and distributor and fire it up.
From Bob Tom:
An additional beneficial step just before fire up is to disable the ignition (remove coil wire at the distributor), remove the spark plugs, and let the starter crank until oil pressure is indicated on the gauge. Replace coil wire and plugs and fire it up.