I bought an entire rear clip from VFN fiberglass:
[url=Http://www.vfnfiberglass.com/site.htm][url=Http://www.vfnfiberglass.com/site.htm][url=Http://www.vfnfiberglass.com/site.htm]Http://www.vfnfiberglass.com/site.htm[/url][/url][/url]
You could cut out for the taillights and paint to match the body. Since the bumper is molded into the clip, you could maybe bolt something substantial but thinner to fit under the clip until you find the correct replacement bumper.
PS I have a 1980 & 1981 Mirada. They both have chrome plated aluminum bumpers. Happy hunting.