The correct tire is a BF Goodrich F78x14 space saver spare tire. Early cars had spares that said 7.75-14, most said F78-14 replaces 7.75-14. They also say "V1" on the sidewall. I just spent a couple months trying to track one down, and the one I got is a little deteriorated on one side but should look OK in my trunk with the good side up!
They turn up on ebay fairly often, we are fortunate the same tire was also used on Ford and AMC spares back in the day. Most I saw turn up on ebay were mounted on Ford wheels and didn't go cheap. In fact whatever I bid someone always bid higher! That is why I eventually settled on one that was OK but not perfect.
GMs used a G78x14 compact spare, make sure you don't buy one of those by accident!