I think you need to take a long, hard, and objective look at that Roadrunner. Yes, from the pics it appears to be a "solid" car, and you say that selling the Duster would cover buying the Runner, but then . . .
Count on needing at least a rebuild on the 413, if not another engine. that one has apparently been setting out "open" for 9 years, probably going to at least need a bore/sleeve on the block, which gets into pistons also. With the wrong motor, and wrong mirrors being known, you have to wonder what else is "wrong" on the car. With the driver's door handle missing, is there mechanism missing in the door as well? I've built a couple of cars up from an empty shell, having to find all those "little things" is getting harder and harder. Just try finding the door latch and lock rods for an E body that are not in the door, and folks won't sell them out of the door. A lot of this depends on what you want the Runner to be/look like in the end. If just a "rat rod" then a lot of the wrong stuff doesn't matter, but if you're aiming for something that looks as good as the Duster, be prepared to spend a lot of $$$, even if you do a lot of the work yourself as I do, parts aren't getting any easier to find or cheaper. Just my $0.02