4.6 4v or 351w
Reply #1 –
The 4.6 out of the '03-'04 Cobra made right around 400 horse, but you'll likely not find one for sale unless someone has a wrecked car..
Most of the 351w that are worthy of straight swapping were either in the first gen Lightnings, or the '95 Cobra R Mustang. As with the Termi Cobras, you'll most likely not find a Cobra R being parted.
The 4.6 will make more power and with more money spent, whereas the Windsor is easier, less complicated, and slightly less efficient.
I'd find a junkyard Mark VIII, and use it's engine as my departure point. It would get a few performance upgrade. Pretty easy to get one in the 300 horse mark. Or just toss a supercharger on it.