I dont have the exact one for a Duster, but this will give you an idea what should be there. Just a little different size and shape will be the only difference. This pic is from a 67 Barracuda I think. Its missing the duct from blower outlet to vent.
Ever think about putting in the "Heated Rear Window" with the defroster grid on the inside of the glass? It was an option on a few dusters, and Darts. You'd also have to install the correct wiring, but if it was mine, I'd rather have that type. (Just a thought)