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)