....and put some anti=seize on the threaded portion. Next time you get brave, find a torch and gently heat the nuts, quench them, and carefully remove them one by one.
This is where I bitch about the old design of AA and EFM concerning the hot water coils. Look at the new style and understand what hundreds of guys learned the hard way. NOW, all you have to do is break it off, cut it off or some other simple method to change a gasket, or replace the coil. You, however should be able to drill out the broken stud and replace as chris or coal berner suggested. I have done the inside out method with grade 8 bolts myself, muttering the whole time about what kind of idiot would design a wear item that requires service to be so difficult.
Good luck, and hopefully you won't have to donate too much 'skin' to the project.