cpl rampage

I am unsure of what spark plugs to get for my turbo build, I know the stock plugs are ZFR5J-11, and the the stickys recommended a step colder per 75 to 100 hp over stock. This would be a standard vitara/eagle build, with a stock head, i plan on hitting 250 - 300.

should i be looking at the ZFR6J-11, or the ZFR7J-11 or even the ZFR8J-11, i will probably get two sets, i just need some help selecting the right pair. I live in Massachusetts, so it gets cold in the winter, and it also gets pretty hot in the summer.

any sugesstions? Thanks