My inventive mechanic had a bolt like that. He first sprayed it real good with penetrating oil several times to let it get in as far as possible. He then took one of those little air hammers like you use with chisel attachments to coax rusted exhaust pipes off and put a pointed chisel attachment which he ground the point off to make it 1/4" flat on the bottom (like a punch) . He vibrated the air hammer with this "punch attachment" on top of the bolt for about a minute and the bolt came out like nothing! If you have an air compressor - it is worth a try.