At this point Juju your best bet is to soak, soak and maybe tap straight on with a hammer, but not much of that. Just trying to loosen up the rusted joint not pounding it into the head. You might need to remove those injectors and get a big pair of locking pliers on the rest of the stud and turn sharply to get them off. At times if all lined up right I would hit the pliers with a hammer to get it to move sharply rather than a slow build of tension. Think about jerking them side to side rather than just pulling on them.
If you still had a nut you could put on there and do a double nut to keep it from moving I would try a sharp chisel on a direct tangent to the stud to deliver that sharp rap to break it free.
Studs can be a bugger to get out at times and exhaust studs have to be the all time worst. Lots of temp changes, rust build up and the like going on there.
best of luck, Keep the bandaids handy. You are going to loose more skin.