The 03 Cobra motors have iron blocks, Ford went to those for long-term durability compared to the aluminum block used on all the previous 4.6 32V motors.
As for fitting a 5.4 32V DOHC in an MN12 car, it would take some major fabrication. I have one out of a 99 Lincoln Navigator, it's a big mutha of a motor. The width of 4.6 DOHC barely fits in an MN12 car, and the 5.4 is approx. 1-1/2" wider and taller again. Overall width of the motor is about 30". I'm envisioning spring towers being cut, and a different type of front suspension, because the stock upper control arm mounting points would be in the way I'm pretty sure. These motors are a few inches wider than a 460 big-block even. Took some measurements of the engine compartment of my 68 Cougar, which will accept a 460. The 5.4 DOHC would not go without sheetmetal hacking on that car either. Oh well, guess I'm going to have to find a Fox chassis Mustang or Fairmont to put this thing. There's some pics posted on Corral.net of a Fox Mustang with a 5.4 DOHC fed by an 88mm turbo, awesome car.
cheers
Ed Nicholson
SCCoO