The rear plug on the passenger side is had to get at. I took someone
elses advice and went in through the wheel well with the tire romoved
and that prove easiest. I use various u-joints and extensions for my
socket wrench to get to them all.
I have a 98 XLT V6 SOHV. I had taken mine to the shop to have done and I
asked about all that you are asking about. From what I was told : The
plugs should remain the platinums, they are a 100,000 mile plug. As far as
tuning goes this is handled by the onboard computer. Just make sure you
have changed all the filters so the thing can breathe and the computer can
properly adjust the engine. This is what I was told, so take it for
whatever its worth. By the way I had mine done at Firestone. If you do it
yourself buy a .99 cent plug gap guage and make sure they are gapped where
they need to be.