Should I sell my Dart and buy a Demon?

I found a 1972 340 4spd Demon for sale and I was thinking of selling my 68 Dart GT.

I am going to look at the car today and was wondering how to tell if it's a "real" 340 car. The engine and trans are gone but the car sits in original (red) paint and such. What is different about a 1972 340 compared to say a 1968? Is this car rare?

Where and what should I look for: Vin needs to match what?

Thanks