Oh Boy...Now we have another list of items to check...
1) Run your thumb nail upward along each cylinder bore and see if it snags hard on the ridge at the top of the bore made from the rings wearing into the cylinder. If your nails do snag, then it is time for a new sleeve and piston set.
2) How do the tops of the aluminum rocker pedestals look? I can't tell, but it looks like there may be deep gouging on a couple...or just optics. If they are heavily gouged or chipped, then you have found your non-ferrous metal.
3) Consider a valve job now that you are this far into it. At your stage I usually pop the keepers and springs off so I can study the face of the valves and check the clearance in the guides. If you do, make every effort to keep the parts together for each valve...You appear very organized, so imagine you would do this anyway! If the valve edges and head seats look smooth, then you are good. If you can rock the valves back and forth when they are just off the seats and you hear a definite click...then the guides are worn. A little motion is alright. "Clicking" is too much.
Guess that's about it for the top end...