Sure you could. The 73 came with a factory A/C option, so if you find a donor car. you could retrofit it into yours.

Mine is a 74 with a single carb as well, as most of the 74s are over here. It did not come with factory air, but my dad had it fitted at the dealer in 1975. It uses the existing vent holes in the dash, and works perfectly. Even the existing fan and temperature knobs are used together with the DEF, REC and FLOOR buttons.

Alternatively, what some others have done is retrofitted entire a/c units from later model 240s that works just like the factory setup. I have even seen a 73 144 that actually had a unit from a Mitsubishi Mirage that works just like factory, but you lose the DEF, REC and FLOOR functions. Astonishingly, it cools even better than the stock unit!

Not sure if an underdash unit works, since your 73 has a full console box, with no room undernath like the pre 73 cars.