The most effective method is going to be to remove the header, and jig it up in a drill press. Otherwise, you run a risk of drilling the first hole off-center, and damaging the bushing as you enlarge the hole. It's vitally important that the original drilled hole is exactly on center, then step-drill it larger about 1/8" at a time. When you finally get to the minor diameter of the thread, carefully use a tap to cut the remaining threads out.
Jerry
