Low Oil Pressure

I have rebuilt an '85 Chevy 305 engine. Rods have been reconditioned, block line-honed, clearance on mains were .002", rods were .0017". All new bearings, all oil galley plugs are properly installed with sealing compound so I know there is no leaks in the plugs. Plug under rear main is also installed properly. Use stock Melling oil pump with 10W-30 Rotella oil. Everything is exactly within specs but it doesn't have the oil pressure I'm looking for. When cold at idle(900 RPM) it holds 50-75 pounds, yet when warm it holds around 5 pounds at idle. When running down the highway it goes to around 30-50 pounds. Engine runs great no noise or anything. Just don't understand reason for the oil pressure to be so low. I am using a new manual oil gauge so I know that it is accurate. I know they say that 10 pounds for every 1,000 RPM is exceptable, but with the engine being just rebuilt with the clearances it has, I would figure it would have more oil pressure than it does. Has anyone run into something like this or has anyone got any sugguestions.