The 96-98 cobra engines are alike, and are rated at 305 hp from the factory. They have the weak(er) pistons and rods, and a slightly lower-flowing intake.
The 99& '01 engines are alike; they are rated at 320 HP from the factory and also have the weaker pistons and rods, but have more flow via a better intake.
The '03 engine is different from both above, it has a forged rotating assembly, iron block (above are aluminum) and has a special intake to take advantage of the roots-type blower.
All are good engines, all are DOHC 32V 4.6L V-8's. They are just designed for different things.
Hope this answers your question.