I would go with one step colder, platinums if you put a lot of miles on the car, just ask for plugs for a 99-04 GT. I only recomend Motorcraft.
2007 Shelby GT500 705RWHP/708RWTQ
2000 Mustang 3.8 A4 TT 450RWHP/475RWTQ 10.996@127 17psi stock 3.8
2006 F250 CC 4x4 6.0L 14.3 @ 93
2006 Mustang 4.0 M5 Vortech 13psi 395RWHP/374RWTQ wife's car