How do I locate the current owner of a 1970 Plymouth Superbird

Some states have written request forms for motor vehicle records or a title search (check with your local DMV), or your best bet try a Superbird registry or classic car registry for any number of car clubs.

they can do that in Britain because a license plate number is assigned to a car for life- in America, really difficult- the above answer is as close as you get- used to be yours?- man I wish I had some of mine back!

I would check car clubs. There is a good chance the owner is a member of a club. Most if not all states won't give you owner info anymore due to privacy laws.