Unfortunately a common issue on Ford mod motors....internet info leads you to believe they fixed it on V10's in '02 but I am hear to tell you they did not. My '03 warped a head, different story, and got both heads done....had I known about the plug issue at the time I would have had inserts put in while they were on the bench...but no. Blew one about 20 miles from home at 4:15 going duck hunting. Thought I blew a rod honestly....figured it out and up-lugged the injector and went home. Never threw a code until I pulled it into the garage to fix it! I used a mid level kit that has a reamer, tap, and guide with nice inserts.
Took about two hours start to finish...took longer to get it on TDC than anything. Intake should not have to come off, on my V10 the back would be hard to do though...not sure on a 5.4.
Some will cringe but I put all new plugs in, and red Loctite on them. What happens is the plugs get loose from heat cycles, then start vibrating and eat the few threads up and spit the plug. My theory was eliminate this and if it does not work, so what, you have to ream the hole anyway to do the job correctly. Fingers crossed a plug does not go bad. Granted I don't see my truck going another 100K miles, has 105,000 on the clock now. I can't believe yours made it that long!
Ford really dropped the ball on this aspect, and head bolt suppliers circa 03.
Good luck at the dealer....around here that is 300-500 job...at least a few years ago.
