The GT40 "P" heads are different from the pre "P" heads. The spark plug angle was changed to move the plug to the center of the combustion chamber. And the exhaust ports were revised to improve flow. Added to that is they have the smaller 1.48" diameter exhaust valves of the non GT40 heads. The spark plugs are the same ones used in the E6SE heads.
When the spark plug angle was changed, it provided a better location for the spark, but it made the other end of the plug land right in the middle of the header tubes. So using P heads with headers is VERY difficult. Instead of the spark plug going in at a 45 degree angle pointing the plug out behind the header tubes, it is sticking almost straight out at a 10 degree angle, putting it right at the center of the tube.
Cast iron. The GT40X is aluminum. I don't think they were ever used on a car stock, but can be ordered aftermarket. Look for the 3 lines on the front. Easiest way to identify them. 4 line is P heads.
There were a very few aluminum non P heads made. But they're few and far between. You'd be better off finding a set of Y's or X heads.
