Cam Bearings can allow oil leakage when engine is warm. Were they replaced (didn't see it specified in post).
My 70 340 engine just rebuilt has 75 psi on cold startup, 20W50 oil.
30 psi at 600 RPM idle when hot, 70-75 psi at 3000 and above.
When I rebuilt the 340, I noticed that the mating surface on the block where oil pump bolts on had a ridge where previous owner had thumped it with a hammer. You have to make sure that the entire mounting pad the oil pump bolts onto (including the oil pump pad as well) is COMPLETELY flat. He tried to use a couple of gaskets to seal it, and did not have any luck.
I stoned it down, and am running stock oil pump and spring combination for the readings above.