Jsut to give people some input on what is the issue with the early generation 5.4L heads and plugs ultimately stems from the fact that Ford machined the head to only have 3 threads holding the plug in place on all the heads from 97-03. In 04, they re-engineered the heads and put more threads there. So, if you undertighten the plugs, the constant pounding the plug takes from each firing of the cylinder can over time loosen the plug and cause it to back out, releasing itself from the head. But, if you over tighten them, you put too much stress on the threads and you can exceed the strength of the threads and pull the threads out with the plug. Either way, not a good day.
As for a fix, the only one that I can recommend is the way that I did which was getting your hands on a Timesert kit. What this kit does is cut out the old threads, exposing more surface area inside the plug well. This area is then threaded with a larger diameter thread and then you slip a collar device back in place which is then locked in place through thread sealant and also expanding the bottom of the collar. This gets you back to being able to use the same size plug, but instead of having only 3 aluminum threads holding the plug in, you now have 6 threads of an aluminum alloy holding the plug in place. As you can imagine, the plug should not pop out now due to having more threads and a stronger material. Down side, the kit costs around $400 if you buy it outright (like I did). But, a new head will cost you $350, if not more, not to mention having to get the new head gasket, bolts, etc (gasket is $50, bolts another $50, etc).
In my case, to prevent a future blow out, I pulled both heads off of the truck, drilled out all 8 plug holes and then sleeved all 8 plug holes. So, now, my motor should never have a dreaded plug blow out again. Besides, that gave me a good reason to port/polish the heads and intake. LMAO.
This kit is capable of being done with the head still on the engine. The "trick" is you need to pack the tools with lots of grease to catch any metal shavings. I am sure you can imagine that any sort of metal shaving left over can be very detrimental to an engine. Doing a single cylinder, the risk is low. Doing all 8 like I did, the risk went up too high for me (hence why the heads came off). Lots of people I know have borrowed my kit and done it with the heads still installed and don't know of anyone that had issues after the fact. Call me a chicken, I will gladly wear that hat. It is your call as to how you fix it.
