What we have done in the past and have done to one of my own motors is talk to your machinist first and ask him what wire can he install in the o-ring groove to deck it flat. When you deck the block, it will be a minimal amount(less than a thousand), the wire will then take up the space of the groove and be true to the deck, and usually it cannot even be seen after it is decked. The wire will never cause a problem and you now can use the receiver groove gaskets your heads have been cut for. Good Luck.