To ensure concentricity and solve the clamps-in-the-way problem, I have on occasion clamped a block of aluminum to the mill table, bored a snug sliding fit pocket for the cylinder head, not quite as deep as the cylinder head, and drilled/tapped a couple of holes next to the bored pocket such that the heads of binding--head (i.e. large diameter head) screws will overhang the pocket. Drop in the cylinder head, then thread in the screws and tighten down so their heads hold the part in place.