My previous car had the block O ringed as well. The machine shop I use also could not O ring the V6 heads, just the block. I had them O ring it for the OE headgaskets, with the O ring just outside of the factory fire ring to help support it. IMO this helped to keep things alive under some extreeme racing conditions. Don't however think that the block O ring will prevent that fire ring from blowing out. I actually blew a headgasket with the O ring, thus causing the O ring to actually smear out of its receiver grove. This is an easy fix though, just use a pick to pull up the O ring at the union point, and pull it out of the grove with pliers. Then you can take a new piece of O ring wire and re-install it. Use a rubber mallet to GENTLY tap it back down into the receiver grove. Make sure you trim the wire ends at an angle so the meet up and look as though it's one continuious piece of wire. When you torque the heads back down it will seat like new.