My NGK plugs are TR5 for the stock range. I run a TR6 that is one range colder. Two ranges colder is actually a iridium plug TRi7 or TR7i something like that...this is from NGK's tech line that is VERY helpfull.
NGK's heat range goes opposite from most brands in that as the number gets numerically higher the plug gets colder. Most other brands get hotter as the number goes higher.
I think the TR6 plug is actually a stock LS1 plug IIRC little bit of useless info.