The box may be missing the heater core which would have the two brass pipes that stick into the engine compartment. You can get a heater core for the box from NAPA or O'Reilly's, etc. The firewall should have another oval hole next to the one shown (that's for the two AC lines). That hole would be for the heater inlet and outlet.
Like Wild said, you need the ducts: defroster, the ones to the center-top of the dash for the AC, and the ones to either end of the dash. The duct to the driver's side is often missing. (your dash needs the center vents for factory AC).
Of course, you need all the AC stuff for under the hood. If you don't have that stuff, you can (1) ignore the AC, get a heater core, and use the box as a heater only;
or (2) you can get a Vintage Air system. If you opt for the Vintage Air, know that it isn't as plug-and-play as it sounds. There is plenty of test fitting and options on how it is installed.
I put a VA system into a Challenger that was an original AC, but the car was using the AC box for a heater for years. Good luck with your project.