If my memory serves me right (& I May be wrong) I think the '62 through '66 Impala had the same body style, so I would look for one of those models.
Another Idea, measure the size of the tank, and go to a junk yard (oops rexyling center) and find one from any other car that will be that size or a little smaller.
You might also wrap the old one in fiberglass and see if that will repair it.
Good luck
The '64chevy was a great car and is under rated.