I'm not sure how you will get it apart at this point. I think I would try grinding the head of the bolt off with a Dremel type tool. Once the plate it out of the way, you may be able to better address the issue.
At one time, Ford had a repair kit for this that entailed replacing the portion that the holes are in without the entire panel. I don't know if this is still available. There was a TSB with the part numbers.
Some enterprising types from CEG found another way to make a repair. After breaking out as little as possible (from like maybe drilling very small holes aroung the threaded insert), they found some "T" nuts in a hardware store of an appropriate size with matching screws/bolts and epoxied the "T" nuts into place.
When you reinstll it, put a little anti-sieze on the threads and you will not likely have this problem again.
There were posts about all of this with pictures, but it was a long time ago.
