I think those "75hp" increase figures are for a 96-98 GT owner wishing to upgrade. The Bullitt heads are basically identical to the 99-01 GT heads. The only real difference is the intake, throttle body, alternator, and accessory pullies. The crank pulley is identical to the GT and Bullitts will benefit from a smaller, aftermarket one.
If you want 280+ to the wheels NA, better consider some new cams, head work, intake work, long tubes, custom burn chip, and a little praying wouldn't hurt.