I have "o" ringed a few big blocks for nitrous applications, and I am not sure if it is necessary for the sy/ty, with the excellent grade fel-pro and gm head gaskets available. If you "o" ring the block, you usually use a copper headgasket, which tends to be a bitch to keep sealed around the water ports. I have seen people use silicone to seal the gasket, but gasoline and antifreeze attack silicone. I use a product, that Bill Mitchell (A well renowned NY engine builder) told me to use. It is called Holomar, or something like that, and I have had good luck with it. Email Jwaller, I think he uses NOS on occasion on his ride, and through his experience, he would probably know. But if you run an engine hot, there is not much that is going to keep the combustion chamber sealed....Steve.