Little or no oil pressure on a big block

I have this 454 engine in a 71 El Camino I just rebuilt. Replaced main and rod bearings with standard size, timing chain,rear main gasket no worked was done in the cam area. I have it in the vehicle. When I fired it up , it sounds awesome but there is very little oil pressure. I primed the pump with a drill and ran it for about 10 minutes. I have very little (nothing) oil coming out of the Pushrods. Oil pressure is at about 20 PSI with the drill but when I put back the distributor and start the engine the pressure drop to nothing. I have pulled the engine 3 times, replaced the oil pan for a deeper pan , a longer oil pickup, pulled the oil filter adapter and looked at the spring check valve and it's OK, changed oil filters twice

What could be causing this?

As always you advice is very apprecated.

Was this motor running with oil pressure before you tore into it to rebuild it?

Is this a tall deck block?

I thought that maybe he was using standard bearings on a 0.010" under crank, too, but I doubt that he'd have 20 psi oil pressure when priming it with a drill motor if that was the case. What I'm thinkin' since he said he had zero oil pressure on start-up is that it's a tall deck block and he's using a standard deck distributor that isn't engaging into the oil pump tang.

Thanks for the info. An uncle of mine came over and we did the bottom end. I had done the bottom end with my uncle 22 years ago (same uncle now) so I went on his word when we put it back together. In response to your question, many years ago (22) when I pick up the car, the crank had been spun so we replaced the crankshaft then with a new one. I know the old ones where standard bearings. The old bearings really did look bad when we inspected them so we did not use a Plasti-gauge to check the bearing clearance. Big Mistake! The car over the years has been parked most of the time and probably had 30K on this rebuild. I had the heads milled, valves and seats changed to use unleaded gas and decided to do the bottom end at the time. I spoke to a few machine shops and they offered to "mike" the crank for me free of charge. I was hoping I didn't have to do another tear down but it looks like I'm headed in that direction. Any other idea before I pulled the engine out for the fourth time. I can now pull this engine in an hour. Thanks Rocky

Motor running had 30 PSI oil pressure before I tore into it.

No, this not a tall deck block and far as I can oil seem to be circulating
Was this motor running with oil pressure before you tore into it to rebuild it?

Is this a tall deck block?

Spoke to a few folks and they all pointed me to plastigauge the crank. Discovered the crank to be a lost less than .010!!!! Pulled the crank and took it to a local machine shop. Turns out it was .020 under!!! I'm having it turn to .030 and and ordering new .030 bearing to match. Thanks for all the help. Rocky