As 7173Vert posted, you should perform due diligence before buying one of these premium-priced collector cars. No matter if a Ford, GM, or Mopar fan, everyone knows someone who has been smoked over a bad deal. Scam deals where the car never existed or a car advertised as a one of a kind Boss 429 that started life as a 6cyl car. The snake-oil selling, white shoe wearing con artists are out in full force. They know that the generation that grew up with these cars in the '70s now have the income to purchase these dream cars and will make emotional purchases. Wise people such as yourself will join forums dedicated to your favorite flavor vehicle. Lots of experience and knowledge on this Forum so hopefully, we can help you spend some (lots) of your children's inheritance!
One of the first areas most people check is the door certification label. It is a good source for Vin, body, trim, and power train content. But you now have to watch out for "Massaged" labels. They are now much easier to obtain than in previous years, so it's now possible to have a label printed to match whatever you want the car to be.
The dashboard Vin plate is another source. The 68-70 Mustangs are riveted to body sheet metal at the windshield. Unfortunately, the 71-73 plate is riveted to the removable finish panel the padded dash attaches to. A lot of people unknowingly pitched both pieces when obtaining a padded dash from a salvage yard. Another caution area.
There is a thin metal tag (Buck Tag) that was fastened to the front of the radiator support on the passenger side of '71 Mustangs. It was a tool used by assembly line personnel that had codes they used to verify components as they were installed. Most importantly it had the complete vehicle Vin. The problem with these is they were very thin and easily damaged. Many were thrown away with damaged sheet metal if the vehicle was ever in a collision. Many were damaged when they were caught on a mechanics tools or arm. They only piece of mine left is the upper portion and the screw that held it in place. Hopefully, this almost 50-year old piece is still on the car.
These next options are a little harder to check. As you already know, there are partial Vins on the upper part of the L&R apron sheet metal under the fenders. Unless there is replacement sheet metal in that area or a re-body the first two and last six of the Vin should match other Vins on the car. There is a similar Vin stamped in the driver side rear of the engine block right below where the head mates to the block. Luck of the draw here. Some are stamped so lightly you can hardly read them out of the vehicle, some so plain you can read them from a distance. The 4speed top loader is also marked with the same partial Vin. The upper part of the case flange where it bolts to the bell housing is where it is located. If a contortionist with a good light, mirrors, and possibly a lift, you might be able to see these numbers. The illustrations show the buck tag, the partial eng block vin, and the 4sp case partial vin.
Hope this gives you some useful info. We're trying our best to help you spend some money!! :biggrin: