I bought a GT500 rear lower valance to install on my 2011, and removing the center snap of the lower valance to the bumper cover (below the license plate) was a bear! Took me over 20 minutes to get it to pull away.
Replacing the entire rear bumper cover and valance would be so much easier! I had this assembly off in 15 minutes!
Post some pictures when done. better yet, maybe create a "How to remove/install a rear bumper cover/valance on a 19xx Mustang". We can then use it in the DIY forum for others.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.