Ok -- so we have oil pressure. That's good. Add the description of the 5-sec clatter for a 100yd run and I think it could be a lifter not pumping up. If so, it could be a relatively easy fix (assuming the cam follower or cam lobe isn't the reason for the lifter not pumping up). I think that's where I'd concentrate my first efforts. While you're poking around, make sure the bolts holding the rocker pins aren't somehow too long, and that the rocker pins are flowing oil well. Getting those all clapped out will make a racket, and a very common mistake is to use screws that are too long and plug the oil passages. Also make sure the pushrods are not broken or bent and that the end caps haven't come loose.
You say you dropped the oil and saw noting unusual -- no swarf, shrapnel, shinies, or chunks. That helps rule out a catastrophic rod bearing or piston failure in progress -- also good.
Oil pressure good and no unusual oil stuff indicates the oil pump hasn't instantly failed. That's good, but it doesn't rule out an oil pump problem any more than these other indirect observations rule out anything else. They just boost confidence.