Frame?? I need one... the whole thing.

The importance of having a vapour barrier on the floor of the garage to prevent this exact senario. If the cost of the work will acceed the price of another 66 Signet with zero rust issues, then buy another. Unless your the original owner with bill of sale and everything is matching number then fixing it might be something to seriously think about.
If you are going to fix it, do it right or don't waste your time.