Nice cars, but they were not COPOs, the option was just regular production. RPO-Z51 I think. According to Scott Windle's web site, 14,463 Novas were made with this option. I have seen several, but I have never seen any of the Impalas, so they are probably rarer. I think I have seen one Vega, but it was a long time ago. Here's a link to a flyer from Chevrolet describing the option.
http://www.classicnovas.net/brochure/74-13.htm