I'm not 100% positive on this but fairly certain that there's an issue with the grill opening on the 03's that's taller than 99-01's. The part I'm talking about is.. If you open your hood and look down at that u-shaped opening where your Cobra/Mustang Grill Emblem is mounted.. The top of either side of that "U" opening sits too high or either too low and creates a gap when you close your hood. (I assume maybe the opening it not tall enough now that I talk about it).
Someone should be able to come along and post exactly what the issue is though so here's a
It will take shaving 1/4 in off for the 03 bumper to fit without rubbing issues...
Any hood besides the 03/04 cobra hood will rub the horns..
If you have more questions ask, I have the 03 bumper on mine, and i have the RR hood...