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Motorcraft or Autolite single or double platinum plugs do best in our engines, there's a couple of good write ups in the tech section on how to replace them.
theres alot of labour involved, esp the back plugs, def. not an easy area to reach into.
hopefully the guy uses a torque wrench, its VITAL the right values get applied to the new plugs. its critcal infact.
for this reason alone i did my own, so i was sure the new plugs werent simply installed wrench tight by some flat rate mechanic looking to get onto the next job asap.
Yeah I'd replace the plugs since the COP boots are old it wouldn't be a bad idea to change them. You should notice a difference in the idle even if the plugs look good. I suspect the plug ground can deteriorate over time.
A $500 quote for $33 of parts and labor to replace plugs is freaking nuts. Figure 2 hours for newbie to change 10 plugs. Once you do it once or twice your looking about an hour start to finish which includes removing the intake tube. Its not a tough job. The #5 plug is the toughest to get to but it is not that bad at all.
I was thinking maybe you change them after the trip. There is a chance that you get a bad plug or break a connector clip or whatever.
If you decide to do it before, make sure it runs good for a while before you leave on your trip. I would get an extra COP handy and bring tools to replace if needed. If you haven't changed your serpentine belt, you should. Keep the old one as a back up for your trip, too.