[Guy]

I have no experience in the metals world, so take this comment for what it is worth. I would always wonder what the material was after your repairs have been made. It may be that all is well in your engine, and this unknown material has been in the bottom of the pan for a long time, and your rod and man bearings show no damage. I think three first  steps are in order here. Cut open your oil filters, and see if any of this mystery material was picked up by the oil pump. Also send the mystery material to someone who does metals testing and identification  by mail  to identify it.. Could it have been from a previous repair? Lastly, if they didn't discard the old oil, get a sample off to Blackstone Labs. This oil test will be a great benchmark to identify and future changes in the health of that Detroit 60.

With luck, I hope you end up with a new set of bearings, a clean oil pan , and answers to your questions.