Dubnali

I've got an LME built shortblock that is 370" 6.0L iron block. It was fine for about the first 1000 miles. It has a PT-88 turbo on 13psi. One night racing the oil pressure suddenly dropped after a WOT pull. It was extremely low @ idle and would only come up to about a max of 20psi. Got the car home, and started it the next day. Oil pressure seemed fine, UNTIL the engine got to operating temperature for a few minutes then, it steadily dropped off to nothing at idle.
Today we pulled the turbo kit, and timing cover etc, because after some extensive reading, I figured it was the oil pump o-ring. We pulled the pump and sure enough, the o-ring looked like it wasn't seated properly. Replaced it, put the car back together, changed the oil etc. and fired it up. SAME PROBLEM!!!
If it were a bearing, it wouldn't build good pressure when cold RIGHT???? Could it be the bypass spring in the pump? Could my block be cracked? I'm pretty sure the bearings are fine, because there's little if any metal on the drainplug magnet, and the oil was very clean. ANY ideas would be great