Me personally I would mill it with a carbide end mill. But short of that, a carbide drill if its really tough, but at minimum cobalt drill. If I am drilling stainless, cobalt drills are where I start.

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......