if you can get in there try welding another bolt on to the top of the broken one. If you can get a good weld and not hurt anything else this can work. its hard on smaller bolts but I have done it before with success. A lot of this working has to do with the heat the welding puts into the bolt and surounding area. Also let the motor warm up any extra heat you can get in there is going to help the bolt free up.

 

You won't beleive this but...
I 've seen it many times. You gotta find an old dude in an exhaust shop. You put a bead of Mig weld on top of the broken stud and then blast it with a hot torch and the thing blows right out, if the guy is good you don't even need to tap the threads.
Not for the weak of heart. Better than taking heads off but not for the squishish.
Gm v6's are bad for this and I've got a guy that does this stuff like eating candy but like I said, not stuff you want to practice on. I've seen my guy do 6 studs each side on a Camaro v6 and not tap a thread. Aluminium heads are even worst, if you get it wrong you got a problem.
Before you pull heads call around see what you can find.
I'm in Nova Scotia so can't likely help...
Best David

 

Get some stainless header bolts. Ditch the crap stockers. This is exactlly why I went stainless on my header bolts!!

 

Where can you get stainless header bolts or stage eights?