The evaporator is set up the same way. This is a reheat system. Air blows across the evaporator then hits the heater core. That dehumidifies the air. Modern cars do this too, especially on defrost.

I would get half warm air and half cooled air. If I were to choke it down to a single core I would lose half my air flow.

Still, the problem lies in finding a 1-1/2" thick heater core that will fit in the box.