The GT-40 irons were the factory heads for the 5.0 liter cobras. They can be found for extremely cheap. The GT-40P's were factory heads for the 5.0 liter explorer engines. They flow better than regular GT-40 irons, but can not be CNC'd as much. These are the heads I run on my engine, which makes around 300rwhp. The GT-40 Y heads are aluminum castings and flow well, but are more expensive than either of the irons. Look around in the classified ads for a good set of any variation. Recently, I have noticed ads in muscle mustangs that advertize the Y heads for very cheap. Hunt around.