I was planning on replacing the dash at some point, so I could do both at the same time. I want the car to be as identical as possible to one that shipped without A/C in it. Will I need to source one of these different heater boxes and/or anything else I haven't got already?

The goal for this car is simplicity - it's got the smallest engine, no cruise control, manual transmission, manual windows, manual sunroof, manual climate controls. It had vacuum-adjust headlights originally but those were removed years ago - I got new euro manual-adjust ones and removed the vacuum tube for the headlights...still need to follow the other one (just capped off for now), and I plan on removing all the vacuum locking mechanisms and having fully manual door locks.