LOw oil pressure on rebuilt motor.

I have a 76 Pontiac 400. It was a boat anchor when I bought it so, I have completely redone my motor twice in the last 12 months. The first time the bearings were under sized so I just got it back running after having the crank polished and new bearing and short block assembled at a local shop. Problem is the oil pressure is fine (50+) PSI at start up but drops to 10 PSI when warm. I have less than 50 miles on the polished crank and correct bearings. I have made sure not to drive the car like this but I am paranoid I am hearing bad rods AGAIN. I used the same (new) oil pump when I pulled the motor this last time as when I built it 12 months ago. I have no windage tray does this matter? Should the Main bearings be fine and should I just worry about the Rod bearings. Or just replace the oil pump?