1970 340 Duster rare x car.

Nothing wrong with an ordinary 1970 H code Duster with a 4 speed.
25 years ago they where all over the place, today they are getting hard to find for purchase. If it's been modified like it sounds yours is, in my opinion that will hurt the value. Missing fender tag, build sheet, and overall condition also play a big part in value. However if the car has correct numbers matching engine and trans that would be a plus.

edit:
Looks nice shouldn't be hard to sell at right price. Would be a lot of money to restore back to original condition. Looks like a nice car to drive and enjoy as is.