Lots of good advise on removal of broken fasteners, here is something that has worked for me on those manifold studs. If they are broken off above the block surface try a stud remover, I got mine at sears several years ago. With a torch heat the block around the stud, dont' heat the stud, you want the block to expand and not the stud. While it is hot give it a shot of PJ Blaster, as it cools it will suck some of this up. Do several cycles of this over several days. When your ready to try removal heat the block around the stud again, don't heat the stud, use a breaker bar on the stud remover and apply a steady pressure, don't over do it, you don't want to break of whats left of the stud. If you don't get it to move a little, repeat the process, be patient and try again. Also it's your call on how much heat to apply, don't over do it if you don't have to.