Local shop use larger headbolts and drills/taps block to accept them. Enlarges holes in head to fit. Did not say what they do for alignment dowels? Seems to me dowels are pretty thin wall? Says bolts are "custom"? I think they are just sourced from another brand/make vehicle? If GM went with simple threaded bolt holes in block, what's wrong with tapping the next size larger? Ya, I know there is a limit to bolt diameter?