Im ordering a few bits off driftworks, and said id get NGK iridium plugs aswell, jus wondering, there is 3 grades on there, which ones should i get??

BKR7?
BKR8?
or BKR9?

Its for a sr20det 180sx

Mods are FMIC, T28 turbo, blitz induction kit, Tomei manifold and exhaust system, car is runnin 260bhp at .7 bar

So which grade plugs should i get???