[Usmellgass2?]

You still have enough stickin out to get a handle on it, Thats a good thing! To avoid maybe breaking it off flush with the head (thats a bad thing!) I would be very careful. Here's how I would do it.
Run 2 nuts down on the stud. leave about 2-4mm of stud protruding out the top of the top nut. Weld the top of the stud to the top nut. TIG or Gas is your best bet. Do NOT cool or quench with water ect. As the stud cools rap gently with a hammer from the top like a nail, not from the side. Do not bend the stud. As the stud cools soak the stud with a release agent like PB Blaster, Liquid Wrench, Krull oil ect. Be careful the top of the stud may be hot enough to flash the penetrante and burn. Hopefully the hammer will help break the corrosive bond and the heating/cooling will wick the oil into the engaged threads. Very carefuly start working the stud back n forth till it will unscrew.

Good Luck!
NE1 else have any ideas lets hear them!