Help me diagnose why my head gasket blew

that's good - you found the issue... and bad - you paid good money and the whole thing has to be redone.
There is a huge difference in machining quality place to place. Magnums are easy to come by but just out of curiosity - if this block has already been run, magged, checked, etc.. Why not use it? Have the operations done (main hone, honest square decking, etc) and put fresh rings in it and go. At least you cut out the 2nd initial cost of buying, cleaning, and checking, and you've already had it bored... Or are there problems there too?