Torquing head nut then it happened

[29closedcab]

Doing the final torquing to 55 lbs on nut number 8 in sequence. That threads in block gave away. Won't hold torque. What are my options? Can I leave go( probably not). I really don't want to pull head AGAIN. Can I remove stud use head as a guide to drill for 1/2 13 tape . Then drill out head to 1/2 so the oversize stud will slide down and finally tap block.