Grab the ubbclassic_test.cgi script from the 6.7 zip, and put it up on your site, with the exsting CGI files.
That's the replacement for the ubb6_test.cgi, and it's perfectly safe to use for any 6-series version... it should operate normally on your server. At least it shouldn't throw any serious "I can't work" errors.
Try it and find out...