i just did this swap with the gt40p,s to my 95 gt the issue is with the headers becuase of the placement of the spark plugs some say to buy the special p headers but i used mac unequal length shorties with a lil work and heating wrap they work just fine
The GT40P heads flow better and have better swirl characteristics. They also have an odd spark plug angle that interferes with many stock headers. The newer FMS (Ford Racing Parts) stainless steel headers fit both GT40 and GT40 P heads with no clearance problems
The markings GT40 & GT40P are cast into the heads in the valve spring area. GT40 heads have 3 bars cast into the front surface of the heads, while GT40P’s have 4 bars cast into the front surface.