The main structure or chassis is the framework that consists of I-beams and columns.  Contained within this structure is a large room that contains the engines and generators that will not be modeled as they are not visible unless the top is removable.  The original model has STLs for the end portions that are exposed.  The remainder of the structure in the original model was a styrene box.  I have chosen to model the underside exposed I-beam structure.  Here is a picture I found of the underside.

And here is half of the underside parts printed up.  The other half is a mirror image.

I will glue a 0.030" thick styrene sheet on top of the center section and I have created mesh flooring for the exposed ends.

Here is a picture of the completed bottom I-beam structure.

I have glued the floor grating to the exposed ends and the center floor which is a sheet of 0.030" styrene.  You will need 3 - 1'x2' sheets.

I am going to print the model using Tamiya acrylic paint.  The vast majority of the model is a light gray.  I will use XF-19 Sky Gray.  Here are pictures of the four radiators that will be placed in the exposed ends of the structure.  After priming I painted the front a flat black, masked off the black and then painted the rest gray.

Here is the main structure with the flooring installed along with floor vents.