On my 500, the Indy heads had O rings in them, but not the block. It adds extra crush to seal the cylinders. I used a Flatout Gasket rubberized copper gasket. Sealed motor well at 14:1, but did have to add some extra silicone around the water ports. Probably because my deck surface was too rough on the block. Anyway, I don't think it is necessary to O ring both surfaces. Cut groove about 60-70% deep of wire diameter.