Before going to the work of pulling the pan, try spinning the oil pump clockwise with a drill motor. Remove the distributor and fabricate a pump drive shaft from the blade of a long flat tip screwdriver about the size of the tang on the bottom of the distributor shaft. It might also be a good idea to install a pressure gauge on a flex hose so you can monitor the pressure from under the hood as you spin the pump. The results might be different from what you're seeing on the dash gauge. If the pressure is still slow to build up, remove the oil pan and the oil pump, and inspect the pressure relief valve in the base plate of the pump. Jon's idea of isolating the filter canister is also a good one.
Jerry
