Unfortunately I have to agree, it is a low mileage car and it is in great shape but it is still just a slant 6 Dart and the color combination is not a very desirable one. If it was a more popular color then I think you could get $5000-$7000 out of it but a clean 70-72 Dart body goes for around $3000-$4000 and in reality that is all you have unless you are lucky enough to find that one in ten people that has the money to spend and wants it for what it is.
I personally like it the way it is and think it should be left alone but I doubt anyone will leave it the way it is, there just isn't any major collectability in a slant six car. It's like my 50 Merc, as a stock body it isn't worth much but with a chopped top it is worth three or four times as much. value comes from desirability, the more desirable a car is the more it is worth.
I had a 72 slant six 2dr hardtop Dart with 42000 miles about 7 years ago and I had it for sale for almost 6 months and only got $1500 out of it, and that guy was going to tear it down and put a big block in it. I also had a 67 4dr Coronet with a 273 with 40000 miles and I got $2600 for that car. Both were as clean as yours but the Dart was green and the Coronet was burgundy which was a more desirable color which made it easier to sell for more money even though it was a 4dr.