Two choices, Ford recommends to replace the head, as when the plugs pull out it takes the threads with it, you may have "some" threads left, but not enough to keep a plug held in there.
The second option is to install a Timesert(similar to Heli-coil, but different), Ford does not recommend head repair, but this can be done in chassis carefully, and there are many Ford techs/dealers and independent shops that have done this with great success, but most will not guarantee anything with it. Cost of doing the Timesert is somewhere around $300 for the kit plus the time of repairing the spark plug hole. Timesert website
Buddy
The second option is to install a Timesert(similar to Heli-coil, but different), Ford does not recommend head repair, but this can be done in chassis carefully, and there are many Ford techs/dealers and independent shops that have done this with great success, but most will not guarantee anything with it. Cost of doing the Timesert is somewhere around $300 for the kit plus the time of repairing the spark plug hole. Timesert website
Buddy