Broke the head off two of the four bolts that hold the link mount brace in my 70 Z. They had not moved in over 50 years so probably lucky to get two out! The captured nuts are inside the channel that is welded into the body. Any ideas how to get the bolts out short of cutting into the channel and replacing it?

Gonna need some pics to even begin to understand the situation at hand. More pics at different angles the better.