You might want to do this as well > Pressurize the oil gallery once the pan is off and see where all the oil is going. I do this on all engine builds before the pan is installed. You can find all kinds of places where oil pressure is being lost. Several ways of doing this. Most high end shops have a oil tank that is pressurized by regulated air pressure. I use a electric driven oil pump/tank built by me for this. Could use a pump up insect sprayer or some other type pump. Places where I have seen engines leak internally. > Cam bearings, lifters, missing oil gallery plugs, bearings.
I test oil pumps as well in a drill press with thin oil to set regulated pressure.
Oil pump sounds like the problem. I would disassemble each and check pump rotor lengths. new one should be longer I would guess, if not the pump is not going to fix it.
Side note: take oil pump off and plug oil hole there before testing.
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