It's not really about coolant temperatures. It's about combustion temperatures, and more specifically how much of that heat transfers into the plug. The R9Ys were for the higher compression engines (340, maybe 273 HiPO I'd have to look) 12s and 14s are on the lower compression engines. You can start with 12 or its equivalent and then if you see need to keep 'em a bit cleaner, bump up to 14 or equavalent.