At the very least, make sure you capture details about where the holes to the heater core, evaporator coil, and the air intake ports are at.

Create a 2D printout or hand cut a piece of cardboard to find any areas that will interfere with the contour of the firewall or dash pieces. I found that I had a sharp turn to make in the heater box as the firewall has a sharp contour around the left engine cylinder bank. A little trimming to the model goes a long way as it will take many hours to 3D print the box, accuracy is important here if you want to build it right the first time.

I used a fan from a Dodge Dakota, which was something I had laying around. The dimensions were similar. I chose to orient the fan's shaft about the vertical axis to make the design as compact as possible. The air has a straight run from the air intake through the air filter to the intake of the fan.

Using the loft feature found in SolidWorks, I designed three outlet ducts that will blow air through to the various VAV units near the vents.