72 demon defrost question

So the story goes i have a 72 dodge demon at some point in its life someone stuffed a 73 an newer a/c and heater core in it, so during are restoration, we added the correct firewall piece,and a/c heater box to the car my question is how does the upper defrost duct connect, am i missing somthing with this box i purchased, i have a parts book and it is ver vage on the defrost duct and piping in help please.