I design HVAC systems for commercial and residential buildings. It is a very bad idea and against building codes to use the building structure for supply air ducts. Building structure can be used for return ducts for some designs. Why, because the large difference in temperature between the conditioned air and the structure surface causes condensation and a lot of it. You know what happens when things are always wet. Mold growth and worse. Properly designed supply ductwork can be designed so it is insulated from these temperatures and just as important covered with a vapor barrier to stop high humidity levels from infiltrating the insulation and causing condensation within the insulation. Use these priciples in designing your system. Furthermore, you must design the ducts so you don't go below the rated air volume of the A/C unit. Otherwise it will fail. The fan in the unit can only produce so much pressure. Make the ducts too small, too many bends, too long and you will reduce the rated airflow of the A/C unit.