Any of you guys ever heard of using a welder to get them out? A mig works really good. It just requires a really steady hand. Did one sunday night on a 318 ex stud broken off just below the surface.
I've done this on body bolts, say for broken or seized truck box bolts on newer Fords back when I was a collision man. Even broken off these bolts will still get a grip thanks to the shank on 'em. If I'm thinking of the same technique you're talking about welding a smaller diameter bolt to what's there and then using a wrench to back 'em out? Been forced to use that technique on equipment back when I was a kid and working for the local town DOT.
Ford has issues with the exhaust manifold studs breaking on the F-series. Spent a lot of time with my head stuck in the fender wells replacing the studs and manifolds on the 4.6s, the 5.4s, and the V10s. Sometimes I would get lucky and the studs would break flush with the manifold and I could back them out with Vise Grips, other times they would break inside the head. I would center punch this with a spring loaded punch and drill a small hole. Because it was usually a long reach in there I would use a Torx-bit socket on a long sacrificial 1/4 drive extension dedicated to the job and tap the Torx bit into the hole then turn it out with a ratchet. The b**** was if the thing didn't want to move and a then out came the torch for a little judicious heat on the stud while trying not to heat the aluminum head. Tricky.
For those of us who've had to deal with rusty exhaust manifold bolts on those corroded 906s or 915s worth saving, I've seen my old man blow the steel bolt out with a torch. The steel turns to butter long before the cast iron gets heated enough to do any damage.