I grenaded my slant 6

I was heading down the freeway, breaking in some new 3.55 gears and the oil pressure went to 0 and there was a clunking noise. So I pulled over. When I tore the engine down the main bearings were all destroyed and the crank needed a .030 grind to make it usable again.

So what caused the failure? The machinest said lack of bearing surface area.
What was the oil pressure before it went to zero? That should NEVER happen. Is there a record of the clearances on those full-groove mains? What was the OA crush dim.?