You've got the bolt out !! GREAT !!
Now... Stop and think. Don't rush anything....
The options are :
1) Drill out the old threads and have a Timesert installed
http://www.timesert.com/html/drainplug.html
Helicoils are shit in my opinion.
2) Drill out the old threads and tap a new thread into it and use a bigger bolt. This is the best option.
3) DON'T OVER TIGHTEN YOUR SUMP BOLT :woohoo:
They aren't meant to be THAT tight. The sump nut on a big GS is only about 32NM. That's 'Nipped' up to most people.