I believe the front fender extensions can go on after the front cover. They look like this o the back side:
Another picture with the top bracket installed:
Here is a decent video of a front bumper cover replacement and you can see how the extensions bolt to the front bumper cover starting around 3:10. Would have been nice if he showed how the extension is removed but no such luck.
Another video showing (starts at about 1:25) how they attach to the front bumper cover and how the body molding attaches:
As for there rear extensions here is an add for them on eBay that has some good pics:
Find many great new & used options and get the best deals for MUSTANG GT Rear Bumper Ground Effect 1991-1993 PASS SIDE saleen 5.0 Foxbody at the best online prices at eBay! Free shipping for many products!
www.ebay.com
This one shows how they attach but I do not see why they have to be installed before the rear bumper.
I will also say that I have never installed or removed any of this from a GT. From the pictures and videos I have seen I thin that the front and rear bumper extensions can be removed and installed if the front or rear bumper is already attached.