1968 Dart question....

I'd be pretty hard to get along with if some guy I made a trade with Son came around wanting to undo a deal I made with his Father.
And if he threatened me with legal action over it, I'd probably tell him to pound salt and run him off.
I get that you want the car back, but the deal was done and it wasn't your car, so you shouldn't go messing with the guy just because you didn't agree with the trade.

Move on.