The first picture shows the Main Duct attached to the opening at the front of the Heater Case. It slides into a clip at the top of the Heater Case and 2 screws hold it at the bottom. The Illustrated Parts Index shows a gasket between the Main Duct and the Heater Case, but the Camaro heater seal kit does not have this gasket.
Second picture - The Left Hand Outlet Duct slips over the round outlet on the left side of the Main Duct. The Left Hand Outlet Duct fits into the curve of the Lower Center Duct and a screw attaches it to the Defroster Distributor. A piece of 2 3/4" diameter flexible hose attaches to the outlet of the Left Hand Outlet Duct and goes to the Left Hand Dash Outlet. Another piece of 2 3/4" hose connects from the round outlet on the right side of the Main Duct and goes to the Right Hand Dash Outlet. When everything is in the car, the right side hose sits on top of the heater case and goes behind the glove box insert. A/C cars have a different glove box that is not as deep in order to make room for the hose.
Third Picture - The Center Dash Distribution Duct slips over the oval outlet at the upper front of the Main Duct and goes to the Center Dash Outlet. The Center Dash Outlet is different for 69s.
I can't find a source to verify it, but I have a thought in my head that the A/C cars used a slimmer radio dash speaker so as not to hit the Center Dash outlet. Many aftermarket dash speakers are larger and won't fit in A/C cars because the Center Dash Outlet is in the way. Maybe that is what I'm thinking of.
Fourth Picture - The various duct pieces are held together with #10-16 x 5/8" screws. While any #10-16 screw would work, the factory used a screw with "tall" hex head. I found these screws at Ricks Camaros - part #HW-96 "BB Heater Box Flange Plate Screws". They're a bit expensive at $3/pair, but not too bad if you are only missing one or two.