Reading the best thing to do is pull the motor, cut the head off the bolts. Then remove the head, weld nuts on. Use heat and penetrating oil. Years ago used to do some VW motors. They had a similiar problem, they sold Elephant studs. Does Subaru do something similar? What does one do if you have to strip the block to get them out? Seems all of my head bolts are this way.
removing stuck head bolts on 2003 Subaru Baja
The engine has to come out to remove the heads, pulling the engine is quite simple on those (not as simple as removing an old Bug engine). I've done 15 head gaskets on those never came across a broken stud. I would imagine if a stud broke you would have to drill it to remove the head, use a stud extractor if enough of the head bolt is exposed or use an oxy/accetaline torch to informally heat the block to remove the stud.