I am the guy 03 Mach 1 is talking about.

1) Have fun trashing your motor with that 6lb lower, nothing over 4lbs should be installed on a truck you want to keep for awhile.

and

2) The threads in the L heads were not fixed until 03, they went from 3 to 7 threads, alot of plugs fly from extra pressure due to pullies and people improperly changing there own plugs.

A good plug for stock to 4lbs is NGK BR7EF's (2 range colder),
gapped to (.036 - .03

Good Day!