You can buy those peices if you want, it's $50 for the 4 pc's it takes (fenders quarters). On my old GT those peices were rusted up pretty bad when I took the fx off, so I took 1 of the decent peices to a local sheet metal shop an they bent me up the peices I need. Not sure waht that would cost you, as apparently the boss wasent there at the time and he just gave them to me
Ive seen alot of ppl use 3m 2-sided tape on those aftermarket 93 cobra fx an that seems to work despite bein kind of a ghetto way of doing things. But from what I gather there is no possible way to use the factory muonting peices on the after market fx, as they do not have the lip to lock it into.
Hope this helps out some