Arty,
Unfortunately you will probably need to pull the engine and get the heads off to drill out the studs properly.
I have done this and it is a genuine pain.
A large drill press is good for this, but it can be done with a steady hand and a portable drill.
IF you are lucky you can drill, step up, drill, step up, etc. until you have a paper thin "threads" left for the stud. then using a dental pick you can pull back the threads until there is enough to get some needle nose pliers on, then just curl the threads back and the stud will delaminate from the head...
Getting the initial "tap" from a punch as close to center as possible on the broken stud is critical.
I know that hind sight is 20/20, but a good penetrating oil and or a dremmel to grind the nut off the stud to begin with is your friend in this situation.
When you reinstall the studs, coat the portion outside the head with aluminum or copper anti-seize (Lamar at American Foreign sells it), a small can will last a LONG time.
(I use aluminum.)
If you get the initial drill off center you may damage the threads in the head, not to fear, you just have to over drill the hole (look up the size needed) and get a stepped metric stud (sold at ace hardware in Varnell or Dalton), just buy good high quality drill bits, they will pay for themselves in spades. You will also need a good metric tap for the larger hole, (the tap will have the correct sized drill bit needed printed on the package if you buy just one tap).
I hate it that you didn't ask before you broke the bolts.
-Note, sometimes when the flange for the muffler is removed there is enough of the stud for you to get a good bite with some vice grips. Put some GOOD penetrating oil (not wd-40) at the base of the stud where it goes into the head, keep as much off the threads as possible. Let this soak for as long as you can wait, several hours is good. Then get the vice grips and make sure they are TIGHT, and clamp them to the protruding broken stud, then do your best monkey arm impression. IF you have been living right the stud will back out and you are good to go. I have a bout a 30% success rate with this, so do not get frustrated if it breaks off again closer to the head (other 70% of time for me).
This is a last ditch effort not to pull the heads (and engine) out of car.
Just remember that there are 4 more bolts just as nasty and rusted on the front of the engine where the heat exchangers bolt on. try some oil and or concentrated heat before you attempt to crack those bad boys off.
Just my $0.02,
KC