use .054. we have already had a discussion on what type of spark plug:
plugs and wires swap out
I searched and researched, made a few phone calls and ultimately went with the Motorcraft SP-498 Platinum Spark Plug.
Also, if you would like, go to http://www.advanceautoparts.com
and get the Autolite spark plugs (Autolite makes Motorcraft spark plugs).
where it asks for discount code, put in the code P20
and you will get 20% off your order. then go to Autolite.com and they have a mail-in rebate form for like $1.25-$2.00 PER PLUG depending on the plug you get (ie platinum, double platinum, etc..)
most are recommending the double plats by the way