It is usual to use coarse threads in cast metals, mainly because they tend to have a coarse grain structure. I agree with mowernut that you should use a coarse thread, but I would make it a UNC thread so the whole engine will continue to require A/F spanners. Be very careful enlarging the holes in the new head gasket: those things fall apart very easily. Don't try to just stick it under a drill press and drill it under power. I'd be inclined to clamp it between a couple of pieces of metal or wood with the right size holes already in them, and use a file (gently) to enlarge the gasket holes to match the clamping pieces. Do a final trim with a hand-held countersink bit.
