If it were my bike, I'd use (or complete) the hole in the fork crown normally intended for a caliper brake and mount the rack using a bracket from that hole. If you ever want to mount fenders, you will need the same hole so might as well have it.

Even on a steel brazed and lugged flat or sloping crown fork, I'd be leery of drilling and tapping into the crown at those points. It sure does beat drilling into the legs but still.....