You could plug the hole if you like. There will be a difference in dimensions as the engine is now angled, so measure from the center of the back plate. The 5-7/8 is an approxamation and there is plenty of cowl incase it has to be extended out. For balance, a light engine needs to be foward on the mount and a heavier engine benefits by being close to the firewall.

Don't be surprised if the thrust angle is not what your setup requires. Again, its a trial and error thing dependant on engine power and prop size. That is why I mentioned to start small, Use washers temporarilly for the angle, fly it without the cowl until it is determined, so you don't have to keep pulling the cowl everytime an adjustment is needed. Makes it easier to do break in adjustments as well.

Once set, mount your cowl without fear of having to move or enlarge the cutouts. The results are better than hacking up the cowl everytime you have to move something. When at that stage, make sure the front is cut as large as the cylinder. A generous cut out for the glow plug makes it easy to access and provides extra cooling. Good luck. Joe