I had a buddy who had me looking at his 340 for low oil pressure as he had a solid cam in it and he had a machine shop build his eng. He had about 10 psi at idle and at 3000 rpm it would only go to about 35 to 40. The machine shop told him that was fine ! Ah I told him no way as this was a complete rebuilt eng with all new bearings and oil pump. Anyway I pulled the pan and put a jug of oil up to the oil pump screen and ran the pump with a drill. Then I saw all this oil leaking internally near the dist running down past the mains onto the ground as they had forgot to put that oil galley plug in near the rear of the eng in the lifter galley. Sounds like the same one you had missing as this you can see when you take the dist out. Since then I have seen this a few other times. Ron