360 heads on a 318, I know it has been beat to death...

Another monkey wrench for you. The head chambers maybe be a bit larger than the published specs. They usually are, for a variety of reasons. The only way to know "for sure", is to have one of the chambers checked for volume (CC'd) before you mill. Also, if you havent measured yet, a fast and easy way (not the most accurate, but better than "1/16 inch down") is to take a metal ruler (not plastic or wood) and lay it on its edge (edge over the piston, parallel to the crank centerline) and use feeler gages to get the thickness of the space between the piston dome and ruler's edge. This should be checked across the center of the piston, in line with the crankshaft centerline. Post that, and the chamber CC measurement, and we can figure out what you need to mill.