best to get an actual measured cc on the combustion chambers. then maybe speak to a/the machine shop to find out how much can be milled to achieve the desired compression ratio. you need to know what gaskets your going to use because they will already account for +/- .020 that they will need to factor in before facing intake manifold etc.
I just checked using your above factors. IF you do have 67cc heads you will be 10.469 with thin head gasket w/gasket vol of 4.180
