Well the front and rear bumper extensions bolt on through the fender/ quarterpanel in place of the stock lx moulding. The side skirts and the front quarter panel pieces are held in place by metal brackets that are rivoted to the rocker panel and quarterpanels. If you get your body kit from a donor car take a drill or a sharp chisel and hammer and retrieve those brackets, they can be made from simple sheet metal but youll want an original or at least a pic of one to know what it looks like.
Also dont forget to get the door mouldings for the conversion as the lx mouldings dont have as wide a "stripe area" as the GT ones do. (my buddy missed that small detail doing this exact conversion)
You’ll need to attach the brackets/retainers with pop rivets and not double tape to keep them in place.
If you’re thinking about installing the GT moldings for the ground effect you mentioned; you’ll need these retainer pieces which hold the GT moldings into place. View attachment 406260
The problem isn’t finding the Retainer’s because you can find them on Mustang Unlimited site