spark plugs

Ok I've got my 520 hp monster going but keep fouling plugs. I am running NGK PFR6J 11's. I am running MC 550 injectors. The car ran perfect before the teardown but I neglected to get the #'s off of the old plugs. have stage 4 port and polish , 50 0ver bore, Mitsubishi R&D ECU japanese board(Found out it is worth about $6000 (yahoo!!!). Any of you other hi-po cars give me some suggestions for heat range plugs you are running. As you know if you do your own work it is a real pain to change plugs.
Please only respond if you have a car putting out 500hp or more.
thanks , Dan