Have you had the original oil pump apart and checked all the clearances? Often lapping the end plate and making sure everything is clean and operates properly is all you need. Especially check your assembled end clearance. Is your releif valve stuck open?
At the other end of the pump is your pickup tube and oil pickup screen. Has it been apart and cleaned up thoroughly? If you have not had the bottom cover off it there is a good chance it is still dirty. Are all the gaskets in place and sealing properly?? Is the pickup tube mounted right and not damaged?
There are also a few oil gallery plugs that can be left out, losing massive amounts of oil pressure.. You may want to study the lubrication system chart in the book, and inspect all the galleries and theri respective plugs... Is this a chain drive engine or a gear drive cam? Do you have the filter hooked up? I think JOhn Barton's rebuild guide over on the G 503 shows the plugs I am talking about.
One thing you could have done while it was on the stand and before your pan was on, is to pressurise the oiling system with an external oil source. If there were any major internal leaks they would have shown up under pressure.
When you have to finish another mechanics work often these sorts of issues pop up....just part of the game, and if you methodically eliminate possible sources of pressure loss for teh entire system I think you will find your problem. I am assuming all your main, rod, and cam bearing clearances are fine.
Good luck!
