Air conditioning can be added to just about any classic car.
Manufacturers that specialize in air conditioning make a kit specifically for most vehicles. Those kits usually include heat and defrost. Some even have replacement factory like controls.
There are some potential obstacles. If the motor has been swapped, you will need to find a compressor bracket for that motor. The vents can be tricky. You can always bolt them on under the dash. The cleanest look is if you can get them in the dash but that’s not always an option. Even if you can, there will be fabrication and possibly paint work involved.
Also, you may want to upgrade your cooling system at the same time if it hasn’t been done already. The AC will put more load on the engine. Vintage cars are already prone to overheating.
These systems are way more efficient than the original units. Even if your car already has AC you may want to upgrade. The original units run R12 gas which is hard to get and very costly. The new units will run R134A.
If you are building a car from scratch, it’s much easier to install AC now than adding it later since the evaporator mounts under the dash and you will need to drill holes in the firewall.
We really like the Vintage Air units. The kits normally run 1500-2000 for the parts plus installation.