The Shelby fascia is identical to the California Special. You can get kits to add the fascia to a GT and no mention is made that the grille would have to be changed. A long way of saying it should fit.
The shelby grill is a complete replacement grill and not a type of overlay. I purchased one off a guy on ebay at an incredible price as from shelby I think they are like $399. The grill is very nice and the same exact one you see in the shelby GT.
The old grill is completely removed. It takes about 5-10 min if you are being very careful. First remove the radiator cover then you use a small flat blade screwdriver to hold up the small plastic tabs on the bumper and the entire grill pops out. You also need to disconnect and zip tie up the fog light harnesses.
The new shelby GT grill just pops right into the stock mount locations in about 10 seconds. It is then a done deal except reinstalling the radiator cover. There are no fasteners other than the pop rivet style plastic ones on the radiator cover.
The grill IMO looks so much better than the stock GT. Yes you loose the fog lights but it makes the grill look so much cleaner. If the fog lights where really going to do any good they should be mounted much lower anyways.
For the $ it totally changes the look of the front of the stang. The guy on ebay was selling them for $150-$200 range and they where brand new in the box. He sells them as "just like shelby" when in fact they are authentic shelby. The grill has the shelby logo and everything molded right into the plastic.
The Shebly grille is a overlay already done for you basically. When I worked at the dealership I PDI'd a couple of Shelby GT's. If you look close , its the stock GT grille with a overlay already on it. Either way, I think the shelby grilles do look very nice, but for my money I'd rather pick up the CDC grille.