i would first try to get a good penetrant in there. they are prob put in dry and will be a real b to get out. if the penetrant has done it's job you might be able to drive them out. then, get the torch out and a good pair of vice grips and do the heat and wiggle thing or weld a fastener to it or you might be able to drive it out.
i would use antiseize on reinstallation for possible later removal
