They still sell the kit that comes with the plug as well as the orings? Man I used to change those things on a daily basis, something about the material of the o-rings does not work well with the oil, they disintegrate.
The ones on the front of the head not too bad but those in back, good lord what a PITA.
2013 XLT S-cab 5.0 2wd 3.31 gear
Retired Ford Master Tech