Clearly you have some issues going on in the photos above. I’m not a fan of filling these blocks as they are venerable to begin with and then you compound the problem with growth and shrink cycles from the thermal dynamics. That will focus the stress in the areas that can move and gray iron is weak to begin with.

IMO…..I would do another build using a stock block as I don’t believe your power level is anywhere near a problem. Don’t fill it, use ARP studs on the mains, and have them in place with all the machining processes, specifically when line boring, cylinder finishing and decking (if done). Use aluminum caps if you think they help….but FWIW….I never have used them and for the same reason as I don’t fill (thermal growth dynamics). The #4 hole may look worse because it detonates the least….IDK…..the carbon on #6 looks like some of it may have been “blasted” out…..lol

You deal is pumping a lot of oil…..maybe detonation has contributed to the rings not sealing and is some of the culprit there, pull the pistons and closely inspect the ring faces and piston lands, that should disclose more. Now that we know your block has been filled and you didn’t disclose any bad news with the other main caps/mating surfaces…..I further do not believe it was detonation that cracked the block. Start fresh, make things right and it should live…. BTW…..who did the valve job? , maybe you should take your stuff elsewhere?….lol….it can defiantly make more grunt than it did….lots of low hangin’ fruit on that tree….lol

Did I understand you are using a stock six pack intake? If so, ditch it and find the wieand or build one similar, as mentioned, it’s not the hot set up for even distribution at upper RPM….another potential source of detonation.