In the sysem for the Sabre, we bought the Super Space Saver from Vinatge Air and then cut it apart and used the components. The small heater/AC combined core is pretty compact. The compact squirel cage fan also lends itself well for use in a '40. I used the demister control (flapper valve) for the control of my outside air that comes from the small NACA ducts on the nose. (The two demister vents and the two eyeball vents are fed air when ever the fan is on. Each vent can be close off individually if desired.) A vacuum dashpot opens/closes the fresh air valve under the control of a solenoid valve with its own switch on the console. The outside air is directed towards the intake for the fan, but not positively connected to it, so that it can be used alone or in conjunction with the AC/heat. They had a vacuum controled heater valve (either on or off), but have now added a thermostaically (and I assume variable) controlled heater valve to their product line now.