Hey,
I'm going to play a couple of roles in this reply.First I'm going to play the Legal counsel role (sort of,lol) Then I'm going to play the role of the hardcore Mustang Geek,Historian. Because I am very intriqued as well. Mostly I want you to take my post as a positive and as a friend,giving some advice.
First you will notice I modified your post, I seriously don't want anybody to get themselves (or the website)into any kind of trouble. Remember what you put out onto the internet can and will come back to haunt you down the road. There are ways to go about things,The best advice has been to try and find out who owns the house and if it is in fact abandoned. Then you can try to negotiate the purchase of the car. Tresspassing onto someones property and initiating "A Garageman's Lien" On a car unbeknownst to the owners is really not too cool,Don't ya think?
It is sad that the car has sat unregistered and untouched since 1973,but always remember there are circumstances. I'll give you a true story one of many that I have. I know a guy who owns a shop outside of which is sitting a 65 convertible.Being that I love all things 65-66 Mustang I had to stop and inquire about the car. He is the original owner ,The story goes like this his son was coming up on driving age(in the mid 80's),They decided to redo the car for the son as a cool father/son project,for the son's first car. Sadly his son died suddenly and unexpectedly,he was playing a good summaritan role and was killed in the process(16 years old).His father was and is still devastated,so he pushed the car outside (half done) and there it sits. As a father I can only imagine(thankfully) what he has felt regarding his son,20 plus years later he can't talk about it. Now as a Mustang lover it pains me the way it's sitting,but what do you do? Obviously I want this car,So every once in a while(I don't line near the shop) I do a driveby past his shop,stop in and see how he's doing.
The above tale is one of many reasons why cars sit. Maybe your scenario is the owner died in vietnam and his family wanted to keep his legacy alive? Who knows,Would it be right to take the car?
For the record I could tell you stories of cars that are sitting(as I write this),Believe it or not I actually know of Shelbies (gt 500 and gt 350's), Bosses,etc. That are sitting in mud and wasting away.These cars are languishing for all sorts of reasons.
Now I'll play the role of a Mustang Geek/ historian. I'd be very intrigued to know the fifth digit of the vin is?It is a harmless number all it tells is the factory equipped engine for the car,If you feel like sharing it here or through a Pm I'd be curious. Whatever you do, DO NOT POST THE WHOLE VIN NUMBER ON A PUBLIC FORUM,This is for your safety only.
Lastly as A mustang historian, Hypothetically of course if one were to have removed a data door tag from a car the right thing to do would be to return it,The rivets are very hard to reuse, unless the tag was removed form inside of the door? If it cannot be reinstalled it should at least be placed on the dash of the car,So that the current or future owner has this valuable information.Ford destroyed all of their records on pre 67 cars. So this is really helpful to Owners and restorers,ask me how I know. I guess I look at this as someone messing with archeaological artifacts.
I would like to see you wind up with this car, But I'd like to see you do it the right way.If you want to pick my brain at all on this matter or Mustangs in general,please don't hesitate. As the current owner of 12 65-66 Mustangs (and some other cars) I know a little bit about this stuff, I recently(about a year ago) purchased a 66 Fastback that I first tried to buy in 1988 ,the owner was not looking to sell,so it took a little while.LOL .Trust me I know a bit about this stuff.