Is it ok to seal the coolant holes for the sparkplugs on a set of old 492 casting heads? The reason I ask is I'm having a problem getting one cylinder headgasket to seal properly because of the .125 angle mill on my heads. The holes have been plugged and re-drilled to 1/8" but one is still too close to the edge of the gasket and coolant is getting sucked into the cylinder. The rest are fine. Do they really do that much cooling of the plug area?