Sounds like almost the same thing that happened to me.
I found a 1967 GTA Fastback (S) code, in a garage, that a 84 year old lady drove into 16 years ago and never again touched it. Brought it home and have slowly been going over it. Has light rust from moisture but over all in good shape for a 40year old car. For yours If it is in good shape I would try to pay less then the lowest NADA price.
Good luck :-)
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