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.