400 is not at all unrealistic if you put an intake, exhaust, boost controller and underdrive pulleys on it you will be between 400-420HP and to get around 500hp add the 550's or bigger, fuel pump, Arc-II or other fuel controller, some nice size turbos (DR500 or DR650), and alcy injection to boost higher(around 18psi) and you should get ya to 500+hp easy