you could always try to simply grind the stud flat and replace the pipe with only 2 studs and nuts. if that fails you haven't lost much other than a gasket and some time. otherwise take the exhaust manifold off and u are gonna have to try a tool known as an easy-out. works like a metal boring corkscrew and goes in with a counterclockwise motion. in doing this hopefully you can extract the stud. if not u may have to drill and retap or use a helicoil. i would first try with 2 studs....good luck