I think you want at least 100-150lbs difference front to rear, so 550/650 or 550/700. You are most definitely going to want more rubber under the rear as well. I have run 275 square, 245/275 in 17's and 245/285 in 18ths. Oh and you are going to want some aero quickly. The cars are so stupid fast with that kind of power (my L33 LSx makes around 375hp and weighs 2600lbs and was scary as heck before getting some downforce on it on the uneven stuff).