Oil pressure

Hi all,
looking for advice as I go along. I had an earlier post on oil pressure and temp, the temp is cured and now I have to deal with the oil pressure. A quick history, I did a rebuild on my 350 pacer/crusader engine. She starts right up and sounds great. Oil pressure when I start is at 50 lb's. when she gets to temp which is around 165 degrees than the oil pressure at idle drop's to next to non existent and at 2000 to 2500 rpm's it is around 40 to 45 lb's. I have pulled the boat out and now am getting ready for the wonderful task of yanking the motor out again. I am running a straight 40 weight in her as someone had suggested and that helpd a bit on the higher rpm's but not on the lower. My kid brother who is a GM tech is leaning towards bearings and others have suggested a stuck relief valve, bad pump and so on. What do you all think? I am open to any ideas you might have.

Sorry about the double post everyone, did not think it went through.

Did you verify the oil was really low using a mechanical gauge?
Could even be the sending unit.
plastigauge the rods and main bearings

make sure the cam shaft bearings are good.

make sure the oil pump is good with the relief valve not sticking, (might be ok cold gets hot and has a problem who knows)
make sure the oil screen is not clogged up, carbon can build up on these and restrict the flow.

make sure the internal oil passages are clean and any oil galley plugs are in place and tight.
Their are limited reasons for why oil pressure is low.

Worn bearings oil pressure can be good with cold oil and when oil gets HOT, it thins out and pressure falls a lot.
Seen this on an engine which had no odd noises, great pressure cold, when HOT, oil light would come on at idle and electric pressure gauge drops to zero.
My idea on that one was worn cam bearings and crank bearings, just not worn enough to hear anything odd.