The trans can be drop out--cheaper and easier then taking the engine out.
If you never tried to remove a broken bolt, I would have the car tow to a trans shop and let them try. They might well come out, I think there is room to chisel it out, least try to turn the bolt.
I would drop the trans and put in a real converter, like a 10 inch that uses the larger bolts. The bolts need lock thread or they will back off. Your flex plate will need to be change. Unless it has around 2.73 gears, the car be happier with a tight 10 inch converter, IMO
