Go out & buy this stuff called Kano-Kroil. I had this EXACT scenario on my 5.8 motor...3 of the plugs did this! I really thought all was hopeless. I went to a machine shop to see what the damage would cost me to have them remove the plugs...the head guy there recommended Kroil and swore to me that if I used it, I would not be back to his shop. He was 100% right. It ended up costing him a job that he probably did not want to do! lol..but it worked! Let it sit & penetrate over night. Maybe even all day, but it WILL come out. Don't waste your time with PB blaster or the like...it won't work. While it is good stuff, its not suited for something this bad...go with Kroil...ya gotta trust me on this!
Roger