A friend had a similar problem (low pressure) with a World block, although it can happen with any block; the oil passage from the galley to the main journal was drilled incorrectly, or the casting had an inclusion where the oil under pressure was escaping in the crankcase. The only way to find it was to put it on a stand and lube it with a drill, pan removed and a remote oil resevoir to see where the oil was going.

If the valvetrain checks out and there are no blocked oil passages, that would be my next step. BTW, they heard of two occasions where an internal leak from a bad block caused low pressure.