Posting photos of the car might help especially if he made any visible modifications to the car.
There is a registry for Panteras at Provamo.com. After you sign up, you can step through the cars using the VIN's for 1973. There is a good chance your dad's car is in there although its color could have changed.
Finally, this is a little far fetched but place a wanted ad on St. Louis craigslist with a picture of the car. It is free.
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