I ended up finding the source of the mystery metal
Many months later I randomly happened to find the cause of all these troubles...the friggin washer just above the cam gear on the HEI! I forget now why I was tearing it down, but I pulled the lower gear from the HEI and found that the washer was a very small and jagged shell of its former self. Didn't really cross my mind until I tore down a different HEI and saw just how big that wash was SUPPOSED to be! Somehow that damn thing got sheared off and cycled through the engine, trashing every bearing along the way.
I spent SO MUCH time and effort and money to make sure I did things the right way to avoid failure, and THAT simple little thing is what trashed my motor.