Just use a kadee coupler box as a jig. Use a pin vise with a drill that just fits the holes in the coupler box. Spin a pin vise a half dozen turns or so just enough to dimple the die cast. Then replace the drill with the proper thread tap size. #50 if you plan to use 2-56 screws.
BTW the problem with die cast is not that its hard, zamac is mostly aluminum. The problem is its actually a bit soft and can catch the drill and break it. Oil is a good idea. Carbide bits are a bad idea because they are very brittle and can break easily. Rarely are they as sharp as high speed steel drills. HSS will tolerate slight off axis drilling without breaking, not a lot but much more than carbide.
Pete
