I have the Cervini's rear bumper cover, and I definitely had to re-use the metal mounting bracket. I also had to do some modification to it to get it to work. Instead of the posts of the bracket coming through the holes in the inside top lip of the Cervini's bumper (like the GT was installed), I had to cut off the lip on the Cervini's piece altogether. I then drilled holes in the top of the bumper (under where the tailights would sit) and down through the top of the bracket. I then used small stainless bolts/nuts to hold them together.
The bumper would not line up without doing this, but I also noticed that ultimate fittiment seemed easier doing it this way. In the end, the thing looks great and nobody knows the difference.
The metal bracket still mounts up to the car using the posts/nuts, they just don't go through the lip in the bumper first.
By the way, CJ Pony parts has a junkyard full of Foxes on-site, and they will often pull off whatever you're looking for and send it to you at a cost (luckily I only live about 40 minutes away).