WIW All Original 74 Duster 360 4 Speed Car?

There are too many differences in ability in terms of tuning and driving to make any real comparisons in of stock cars driven by different people. I'd wager the same driver on the same track with the same tuner will get similar performance from both cars when optioned identically. 340 wasn't all that great, and 360 isn't all that bad.
In terms of value - I like the car, it's "had work done" but is a great piece to have and drive with a ton of potential. I don't feel it's a $20K car, but if YOU think it's 20, or 25, or 15, that's fine. As a buyer you have to value it on what the market might support, and what you will get out of it. I place a high value on enjoyment, but you have two other cars you also enjoy so that value is at best 1/3 of what it could be... The fact it has not sold in years since the new owner bought it tells me they are far enough above market that he's out of touch and holding to hope and dollars. He bought it for income. You will never win out over a seller that's only doing it for money unless you put no value to spending it. It's your money. If it's worth it to you buy it and go enjoy it.