I have used copper head gaskets and there is no problem with sealing the combustion chamber. Sealing the water passages has allways been a problem. Almost every engine builder that has used copper head gaskets will have their preference of what to use, silicone (in many colors),hylomar(brush or spray),copper coat and permatex brown sealer. No matter what you use water will seep out somewhere. They also have copper gaskets that have sealing inserts to seal the water jackets. If you have a block with o-rings you can either keep the copper gaskets and contend with seepage problem. Or can have the block milled. I have milled quite a few blocks with the o-rings to accomodate a Cometic or MLS gasket. If the groove has been machined at the proper depth for the thickness of the wire, the block can be milled with the o-rings. I have a Peterson segmented grinder and with light cuts and a slow feed the o-rings can be milled flush with a minor effect on the block hgt. I have also pulled the wire out of(while milling) the groove and have had to mill the block to remove the groove. I have also filled grooves with "Marine Tex" in desperation when a wire pulled out after reinstalling the wire and was spit out a second time. It held up, but that was last result. If the block was o-ringed, I woud advise a receiver groove. Copper head gaskets were the only option before Cometic or MLS and are still a viable alternative. If you have Aluminum heads the o-ring should be install the head, but that is not the case. If you are contemplating the cost. The price of the gaskets are probably comparable. If the stainless wire was removed and copper wire reinstalled and filed to accomodate the MLS gasket, machining a receiver groove might be comparable. You can not cancel out what has worked for so long. I agree ,if you were starting from scratch - use aCometic or MLS. You have a block with o-rings, receiver grooves are a suggestion - not a necessity.This was batted about on earlier posts and it was stated by a credible source, The only problem Cometic or MLS is if there is a detonation problem the "block might burn through before the gasket". The gasket becomes the fuse. You now have to weigh the alternatives,torch the gasket or the block.