It took some digging but this is what I bought. They fit very well on my wheels. I Think the mustang hub is 70.3 mm so the 70.5 mm is i tight slip fit. If you have a way to measure, you can confirm the 73 mm (2.874 inches) bore diameter on the wheel. I bought the metal ones and they have worked well since i put them on in June.
http://cgi.ebay.com/ebaymotors/Metal-Hub-Centric-Hubcentric-Rings-73x70-5-73-70-5-/220712066157
Hope this helps.
http://cgi.ebay.com/ebaymotors/Metal-Hub-Centric-Hubcentric-Rings-73x70-5-73-70-5-/220712066157
Hope this helps.