This is a problem on 4.6's you here about all the time. I blew a spark plug out about 2 months ago on startup. Just get someone to install a helicoil. It cost me $350 labor and parts and thats including a new coil pack. The spark plugs on a 4.6 only go in about 4 threads before they stop in the head. They changed it i believe in 04. 2 of my other mustang friends have blown out spark plugs too. And thats on n/a motors not during racing.
Josh
Josh