I haven't done the plugs in the 5.4, don't see the need at 149K. But my go-to dealer is $140/hr plus parts and the repair kit - easy $750 plus with a couple of breakages. The advice is "repair the cylinder that has a problem, they'll run 200K well enough". So far, that's how it works. Had one coil replaced, didn't hurt the plug. I know the Master at that dealer and he can get them R&Rd, typically without breakage - but it takes time. Time = $.