I make a ring of wood that matches the opening on the cowl when bolted on like the propeller. Place your cowl over that and it will dictate where the cowl needs to be mounted. Leave about an 1/8" for the thrust washer to protrude in front of the cowl. This way you get the cowl centered over the prop shaft. Once you get the cowl mounted you can go back and make any holes you need like for the muffler. As suggested above.