I'm afraid if you check www.splitfire.com Copper Core classic plugs are resistor plugs. Everybody says non resistor is the way to go. The dealership can provide them for less than platinums and you'll have the recommended plug in there. Also the exact proper heat range may be coming into play with other than recommended H9DCO.
I worked at a car parts store in the '80's when platinums came out and in Bosch plugs one platinum plug would replace up to 4 different heat ranges in the copper core. We had plenty of people bring platinums back because they wouldn't run right with the application being used.
Dan