You can buy the ORB cutters from MSC for around $200. I had to have this one ground so it would also cut a recess for the plug's head to make it flush to the machined surface and not impair the water pump gasket seal or plate between bellhousing and block.
The entire setup pushed the limits of my mill Z travel, I actually had to shorten the pilot nose of the cutter by 1/16" to get clearance for the block to slide under it. I originally considered hanging the block off the front side of the bed by a main cap (that's what I originally cut the girdle up for) but I settled on trimming the cutter a bit... where there's a will, there's a way.