Last head I did I had one bolt that just refused to come out before the head stripped out completely. I grabbed a 1/2" drive drill and a bit just about the same size as the head bolt shaft. To catch the bits I packed some old rags near it a set a magnet close by. A little bit of time drilling and the head of the bolt was gone The head was free to lift up over the headless shaft. Once the head was out of the way, I buzzed the top of the left over bolt shaft with the grinder on two sides to get a grip on it. It came right out with a few taps of a hammer and turn of the wrench.

If you need to replace bolts when you are done, you can get a set from www.headbolts.com for fairly reasonable. I have never ordered from them, but have not heard of any complaints about their service or parts.