I've never seen a plastic model kit of 289 Cobra done in any scale larger than 1/25th, so you might have a real treasure there.
Ernesto:
Here are several links on Ebay to the AMT/Ertl 1/25th scale 289 Cobra kits. There are several different versions with different boxes and molded in different colors, but inside they are all the same kit:
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=2583
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=2583
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=2583
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=2583
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=1190
Have fun!
Russ