Guys, there's been tons of coverage for this subject.
The 03/04 cobra HOOD *WILL* fit in a GT bumper with no modification.. that's been proven several times, and in fact there are threads with pictures both here and at modular depot. You will have a slight gap between the pieces that run verticle on each side of your grill between the hood and the bumper. You can unbolt it all, and drill new holes further toward the engine compartment to pull the bumper upward, but it's not required to fit.
Furthermore, there have been tons of threads about putting the 03/04 cobra bumpers on, and in fact earlier, I was looking at an 02 or 03 GT on ebay with the stock hood, but 03/04 cobra bumper. So the 03-04 cobra bumper fits. I would imagine as with any modification that involves replacing an entire panel, SOME work may be required, but as to how much, that can probably be found by doing a search here, or on google.