Scored Main Cap Bearing - Ouch

If the metallic material that you think you are seeing in the oil when drained is like a silvery sheen on/in the oil, that is not likely bearing material. It is likely one of the assembly lubes or aids, and that super thin grey coating initial coating on the bearings themselves. That is something I see in pretty much every rebuild.

You don't have to 'fly blind' on this matter: Keep that oil separated and send it to an oil lab for analysis. If you did not drain it into a squeaky clean container, then it may be contaminated and not give good results. If so, then run another 100 miles of moderate driving, then drain the oil into a clean container and have it analyzed.

They will be able to tell if there is bearing material in there. You can have it analyzed a couple of times to get a much better idea of what is going on.

If yo do the kerosene or diesel flush, then run cheap oil for 5 minutes or so after wards and drain that out once it warms up a little. Basically, use the cheap oil as a flush for the flush. I have no idea what the Amsoil flush will do to seals, so personally I would just use diesel or kerosene, not running, to flush the pan out. Pour some through each head with the valve covers off.

IMHO, drive it with slowly increasing RPM's and hardness. Change oil at like 100 miles, 300 miles, and so on. Have it analyzed a few times to see where this is going.

You may well be fine, since you seem to have found the culprit. The bearing material that came out is relatively soft, not steel. The very limited scratching of the crank journals attests to that.

BTW, good deal on the seal !!!