So this is a whole new topic, and not very Datsun-specific.

B310 has factory airconditioning. But if your dashboard has odd controls (heat separate from cool) and the A/C vents are below, not part of the heater vents, then it probably is aftermarket.

You generally either need to replace the evaporator or have it purged before recharging the system. I suspect the experts in Phoenix know more about it. Maybe the air is dry enough there to not worry about it.