Do you mean like a anti-theft cable so it wont turn at all, or a cable that attachs cap to the body so it comes off but hangs by the cable and harder to steal? Either way I want to do the same.
Rodney, Those cables were a factory item when the Mustang was new and "Collectibles" like the running horse fuel cap were a prize among the "Midnight Supply" people. In fact my 67 had the cable. Not sure how hard it would be to find the hardware to attach a cable to your cap and car body with out damaging either and still look good. The cable on my Dad's 66 Mustang was a high strength hard to cut cable and plastic coated so as not to scratch anything, so that's another thing to consider.
Don't know how much you wanted to spend, but I would look at Don's (Ohio Mustang Supply) site. He has a listing for the security cap for a 71-73 and is also a forum member and Vender and will make sure your taken care of. http://www.ohiomustang.com/store/order_page.asp?itemid=2051
My cap is in good shape and it does not make sense to purchase a new cap just for the cable. I would however like to know the length so I can make my own. Just do not want to leave behind after fill up.
You need to make sure your cap has the loop for the cable and that your filler flange has the screw with the loop. Then get a foot of stainless steel cable from the hardware store and a crimp connector. Cut the cable to the approximate size shown in the link below. For double peace of mind I would also solder the connection with silver solder (not next to the open filler, though).