I think the flexplate steel makes it tough to drill. Personally if I had to do it I would be milling the hole larger with a carbide end mill on a cnc. My cnc spoils me..........

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