The first thing to do is to plumb a direct-reading gauge (not electric) into the port beside the distributor. If the pressure of that gauge matches the electric, I'd recommend pulling the oil pan and inspecting the pump. Depending on the vehicle the engine is installed in, the oil pan may or may not come off with the engine in the frame. So far, you haven't told us what vehicle we're dealing with. Once the oil pan is off, the pump removes from the rear main bearing cap with one bolt. Melling makes a high volume pump that will maintain pressure at idle even with a race-clearanced engine.
Jerry
