Not. The only way to get 20k for a '73 340 4-speed Duster would be to time travel back to 1973, pick it up off the end of the assembly line, and bring it back. All original, no miles, all the documentation. Then maybe. I think $20k would be the absolute top of the market, it would have to be perfect. This one is definitely not perfect. Nice car, but if it's not a real 340 4 speed with matching numbers then its $12 to maybe 14k, depending on what a closer look shows.
The problem is if you have $20k burning a hole in your pocket to buy a classic mopar there would be a lot more interesting cars in your price range. Like 72+ Challengers. This one won't break $20k. Maybe not even $17. It's definitely not perfect, lots of little things going on that you could pick apart- wrong dash for one. But a running, driving, matching engine 340 Rallye with a fender tag...
