How can I find previous ownership/location details of my car?

There are some pretty strict privacy laws here in the USA after 9-11-01. Even law enforcement officers can't just run VIN numbers for themselves or car buddies without getting in trouble if they get caught. They can supposedly only run them if they have probable cause or other investigative reasons.
You may check with a private investigator in Australia to see if they have any means to look it up for you. I had a private investigator look up an old tag number that I found in a car that I bought, and his help ended up getting me pointed in the right direction for some paperwork that I wanted for that car. Even if the private investigator in your country can't get access, he or she may have a peer in the USA that they have worked with in the past that can help. Good luck to you, and you did a beautiful job on your Valiant.