What a mess. It sounds like the black car has more legitimacy to have the stolen vin tags on it than the car it was stolen from. At least from a title perspective.
I'm not sure what year the white shelby was stolen, but if it was a long time ago, it is probably too late to do much about it. That is, unless, you have a good friend in law enforcement that could pay a visit to the current vehicle owner and put a scare in them. Or possibly hire a repo company to take possession of the black car due to it wearing stolen, original vin tags.