When it came time to change plugs on my 2001 F150 with the 4.6L Triton, I had the dealer do it (about 105,000 miles showing). In CA the service provider has to give an estimate before starting the job and the customer has to sign off on that estimate. For the 8 plugs, the estimate was $400. The cost actually was much less (~$150) but the service writer said that they have to protect themselves because it's pretty easy to bogger up the threads in those aluminum heads, especially in the two plug holes closest to the firewall, and it's pretty expensive to repair with an insert if the worst happens while changing plugs.
There used to be a guy here in CA who had a mobile repair business just to handle Ford Triton spark plug thread jobs in customer's driveways. Apparently he could make a decent living fixing screwups that his customers caused trying to DIY those plugs.
Be advised and good luck