I have seen lots of Ford 9" axles drilled with that same hole. Doesn't seem to bother the Mustangs the race at Willow and they reach some pretty high speeds.
I think you will have a very hard time drilling the axle. I would leave it alone. It is probably hardened 4340 and you would be very hard pressed to drill a 1 inch hole in the stuff. Even clamped in a CNC it is miserable stuff to machine.
Hope this helps...
David
