There was a 67 dart that sold locally that was almost an exact build of yours. It had racing seats, fenderwell headers, 440, 8 3/4 tubed, roll cage, etc etc. I know its not the same year, and your car has a better body, but I find that cars that look more like a race car sell for less than a car that is done out, but still looks stock ( interior)
the 67 dart sold up here for $12,000, and he was trying to sell it for Months. He started at $16,000. and this is up in Canada, where cars are usually high in price.
your car as is, around $12,000. But if you put the interior back to stock, I think you could get $15,000 or more.
Got to remember, if your not a race car driver, or going to drag race, than you don't like the roll bar look, and racing seats arn't comfertable.
Phil