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.
1998 GT - CAI, trickflow intake, 3.55 gears, O/R x-pipe, flowmasters, short throw shifter. 145k miles
1995 GT vert - 1 of 36 5.0 Appx. 450,000~ factory motor miles *totaled/parts car* RIP..