gt40p heads have the exact same intake and exhaust cc that stock heads do...BUT, the combustion chamber is angled differently and creates more compression than stock or gt40s. Honestly, I would take either or depending on the price, they are about the same hp gain wise. The gt40p heads have different angled spark plugs that make the sn95 stock headers useless though... Not sure if this is the case with a fox.
1995 mustang gt vert - 1 of 36 5.0, AODE, 2.73 gears, Pacesetter O/R x-pipe, Flowtech Terminator mufflers. Appx. 410,000~ miles. (honestly not sure..)