Is this a street engine, or an all out competition engine?
If it's for street use, then why on earth do you need to "o" ring it?

If it's an all out competition engine, then pull the whole bloody thing down & do it right.

Ferral Errol is right about the rag plug etc. but, if it's a STREET engine, & if you are having head gasket problems, then fix the ACTUAL cause rather than wasting your time & money on "O" ringing the block.
Chris