Hard block???

I don't think it's the machining as much as it is the torque plate installation immediately after filling and leaving the torque plate on overnight while it sets up. It should be okay if you torque the main caps to spec and have a torque plate tightened up to what your machinist set them to on each side of the block while letting the filler set up overnight. Your goal is to try to duplicate the distortion of the block from having the mains torqued and the torque plates on while it was being machined so that when the filler sets everything will be held straight, round, true, square, etc.... It's the same procedure before machining one so as to "pre-stress" the filled block to its normally assembled condition, so that the drawing forces of torquing everything down aren't pulling against the filler.