Agreed, something happened but danged if I can figure it out. Maybe it was just a spell of bad luck. My plan is to put this back together as carefully as possible and see what happens. I did check all the oil passages when I gave the block a bath, and all are good. I have slobbered plenty of assembly lube as I go. I plan to leave the plugs out of it when I start the break-in, and spin the engine to get the oil pump working and the galleys filled. I will fill the pump with vaseline to prime it.
I'm putting in new rings to make sure there's no issue with the oil/rich mixture/carbon business and I may swap out the umbrella valve seals for a set of the positive ones. I shall report on things moving forward.