Ok, if you are still needing some practical input.. I wrote on this exact topic several years ago - Unfortunately, I have had to do this a couple of times on several Z's over the years. Both times what I did was remove the exhaust manifold to make room. I then used a torch to heat the area on the Head near the broken stud - not the stud itself, just around it. Now for the 'fun' part - Using a small, very sharp chisel and a ball peen hammer - you begin tapping (with a corner of the chisel) counter clockwise until the stud begins to back out. Re-heat every once in a while. The stud will back out - You have to be real patient and take your time. Be careful not to nick the Head, let the chisel bite into the stud as you work it backwards. When you are done, be sure to blow out the bore with compressed air and use Anti-Seize on replacement stud.