Posting a Cobra or a GT40 find like that is risky at best.
In the past people have shown up with a bolt, a bill or sale and a lawyer claiming that they own THAT car and demand you surrender it to them.
EVEN if YOU own the real car, it's in your possession, that doesn't guaranty that there is another one that already exists.
Best to keep the discovery to yourself. Many states on cars that old were non-title states and the car could be registered without a title and a makeshift bill of sale.
When the car was worth $150, that wasn't so much of an issue. Now that value is somewhere over $500,000, and rats appear out of nowhere like post Civil War "bounty hunters".
So far, to my knowledge, no one has been murdered over these scenarios but who knows what will happen tomorrow? It's dangerous out there.