Richard;

You have made a judgment call by stating: "there is no way to get a new one made.
" ...I would take the key remains to a locksmith who has the correct blank (or also bring this with you), and let him make a copy of the key or at least let him make the call that he cannot...an experienced locksmiths can usually work it out...otherwise he could "make a key by impression" (another technique of making a key for a lock when you have a key to copy, other posters touched on this).

Once you can turn the lock cylinder to the Ign ON OR Accessory positions and withdraw it, you may very well be able to replace it with the one allocated for the '67...

...you are obviously aware of the benefits of adding a Pushutton Start Switch to prevent reoccurrence...

Additional suggested reading: http://www.sw-em.com/service%20notes.htm#Body

Cheers