ive welded nuts to the studs before its a simple process
1 remove manifold
2 spray stud with pb blaster
3 spray stud with more pb blaster
4 with a heat gun or propane torch evenly heat up the area around the broken stud
5 have a buddy hold a nut (that has a smaller inner diameter than the stud) over the stud flush with the head and fill that inside of the nut with weld
6 let the weld nut and stud cool
7 when the weld is COLD evenly heat the area about 1in from the stud
8 while the head is hot put a wrench on the nut and try to back it out.
good luck hope this helps this method has done the trick for me twice

 

Got it out. Took to buddys shop, he welded a nut to the broken stud and just backed it on out. Made it look like easy.