It is also possible to tap the composite mounts to whatever thread needed. Done that a few times. Use a smaller tap drill for fuller thread. I have threaded the hole through and put a nut on the bottom. It will pull up on a rounded mount and won't vibrate loose. 4-40 should be fine for a .40 sized engine. The screws will will pull into the mount and won't shift. Doesn't take much to hold a .40. Two 6-32s installed diagonally would probably work.