It is my understanding from your posts this engine had good oil pressure to start and only started doing this after you started running it hard for the first time.

Two things could have happened. The bottom end is comming apart or the oil pump relief valve is stuck open. No you don't have a relief valve in the block. This relief is built into the pump.

If you never had good oil pressure from the start then other comments make sense.

Did you have metal in the oil when you did the oil change? If not I would rule out the bottom end going south. Stop running it till you have this figured out.

I personaly would drop the pan inspect the pickup then change the pump.