I have/had pictures of a sectioned 60 head showing the deck thickness under and around the eyebrows. Take all the eyebrow off and use a set of valve springs from an 80/820/830/840 iirc they fit and are much heavier. The problem is with the open chamber combustion chamber design. There is no squish or quench area which reduces detonation. Unless the there is some way to make a flat potion of the piston approach within 40 thou of a flat portion of the cylinder head.