What about the E3's with diamond fire technology? Im not sure since I have not looked them up for the supra, but what I was told from a friend that races old school muscle, they work like a charm and have really nice power increases.
e3sparkplugs
Update: They do have them for all years/models for the Supra. Interesting comparison that the series "Horsepower" did between the E3's and I'm not sure of the other brand.