I don't know off hand how much you can take off but it can be calculated by the cc / volume using a calibrated baby bottle or other measuring devise and liquid. Plug the sparkplug holes and fill heads to leveled gasket surface. Dump each head into the measuring vessel. Now you know the volume of each. Skin cut head gasket surfaces to volume required. Doing the skin cutting in this manner eliminates differences in front and rear head combustion chambers. Just don't get carried away and remove to much material. Check volume often.