As I understand it, most of the reasoning is around brake cooling ducts, routing of cooling lines, radiators, etc but I haven’t taken one apart or read up on it that much to know for sure. They are slightly wider than a GT/EB as well. But basically from an OEM parts standpoint, the Shelby’s parts are bespoke from the fenders forward sans headlights (the headlights are the 15-17 design).The same goes for the OEM GT350 fenders as well I think - if you were considering those too. IDK where the proportions differ, but I think if you wanted fenders that look like them, you would need to go aftermarket for "GT500 or GT350 styled" fenders that fit the regular GT/Eco Boost/V6 models as the OEM ones will not fit correctly.