Another interesting fact, factory installed AC in the 60s and early 70s used an A/S proprietary design that did NOT use any of the existing vent openings. The factory bracing and wiring expected this proprietary type unit. I wish I could remember the name of the manufacturer (seems like they were called Bay Breeze). (added...I just remembered the manufacturer was Armstrong)
The AC was in two sections. The outside section contained the compressor, the condensor coil, condensor fan and the fan motor. The inside section contained the evaporator coil (part that gets cold, drip pan, and the interior fan blade. During installation, small holes were cut thru the top and inside skins for the refrigerant lines to/from the evaporator, the fan motor shaft (motor had a short shaft on one side used for the topside condensor fan and a longer shaft on the opposite side that extended into the interior for the evaporator fan blade), and a hole for the electrical. The topside fan exhausted air vertically while the inside fan exhausted to the edges, but both were blade fans not squirrel cages.
The installation could be done in the field, but NOT as a DIY project since the refrigerant connections had to be field soldered and then the unit vacuum purged and finally charged with R-12 refrigerant.
I am not sure when A/S moved away from this design, but I DO know that this was the way it was done during that timeframe as my parents owned several A/S during that period. These A/C units had a very low outside profile and worked extremely well. The lower profile outside was slightly offset by a slightly deeper unit inside, (compared to the Dometic units A/S uses today). The topside housing was a custom aluminum design that blended into the overall aerodynamic A/S look. I remember how quite the units ran, especially on low speed (more than enough cooling at night).
I know that this does not help you with a new install, but thought a bit of history might be interesting.