Is it a steel bolt (as opposed to aluminium) ???
With the amount of thread still protruding, I'd be tempted to simply wind a nut onto it finger tight, then weld the centre of the bolt / nut together, and use a socket on the nut to wind the lot out of the hole. The heat that you would impart through welding will help free up the thread and allow it to come out easily enough, provided you attempt to remove it while still hot.
(Clamp your welding earth lead onto the outside of the nut and weld through the centre - a Mig would be best if you have access to one).
Make sure you disconnect your battery/s before attempting this though. ;)

