340 exhaust removal

Take the top end off, you are going to be doing it sooner or later.
If you're changing heads anyway, why bother to try to remove the broken off bolts/studs in the car?
You can do that much easier when the heads are out and on the bench.
Use a torch to loosen the bolts in the first place by heating where the threads go into the head, get it good and hot there.
If the engine still runs, warm it up to operating temp. and loosen the bolts while the engine is still hot.