you dont really need to go colder plugs untill you drop pulleys. for the 3.5 you would need to drop 1 heat range (al 104's) if your not droping pulley sizes i would go with 605's. the hotter the plug the better performance. but the colder plugs reduce KR that you get with a pulley drop. also get some exhaust mods to go along with the pulley ie. downpipe and ported manifolds or just headers