Whether you can easily add one to a '30-'31 Ford depends on whether you have a cowl gas tank as well as what ever else is in that small space, like wiring and gauges. If you have a good clear area, the a Vintage Air Gen ll Compact will fit there and be virtually hidden from sight.
I put one in my '31 roadster then took it out as "dumb" for an open car. It was complete, ran but was never charged (no, it's NOT for sale).
You need the radiator condenser, a Sanden compressor, a bulkhead fitting, the evaporator, hoses, a dryer and many fittings along with about a weeks worth of time and probably a thousand bucks more or less for the material. This is what mine looked like:
I have a heater about the same physical size there now, also a Vintage Air so the air ducting is about the same though the hoses were easier to make up.