[Will]

Ditto the heat suggestions. Get an oxy-acetylene torch, a regular propane torch isn't going to get it hot enough.

Get it as hot as you can, glowing if possible then let it cool completely before trying to pry on it. Might take several heat cycles but it *should* break loose.

Do you trust yourself to hold a drill straight & steady? Try drilling out the center of the bolt. If you drill enough of it out it may weaken enough that it'll break off when you pry on the head.