All the usual tried and tested methods mentioned above. I'd add HEAT into the equation.
It's amazing what a good dose of oxy/acetylene will do to free up a stubborn bolt.
I'd try centre punching the thing and drill it before cycling maybe 5 goes of heat and cool. Don't pour water on, you'll possibly make things more brittle. Then get some form of bolt into the hole so it can be welded on. A good socket/long lever and fingers crossed it'll release.
TBH, if you've got it off the car, most machine shops will have encountered your problem and should be adept at getting it out (often for beer money).
Good luck..