When you drill the bolt, you still have to insert an "easy-out" and twist the bolt out (you don't need to drill all the way through the bolt, right? ). You have to apply the same amount of force still, just to a different tool. With the head ground off, the part that was actually frozen, the interface between the back side of the head and the face of the harmonic balancer pulley, is no longer there. After grinding, insert the puller, which you will have to do no matter which method you use, and use it to slide the pulley off the shaft.
Sounds like you're almost done drilling so....nevermind.
Tom