I had never done any 3D CAD before so I started with Google Sketchup
. I decided it would be interesting to learn, and it had the added benefit of being FREE and running on my mac. (If Autodesk 123D had run on my mac that would have been a better choice.)
It turns out that Sketchup was NOT designed for this type of work. I wasn't going to pay $495 for the pro version (which had solid tools.) I should have just gone to Techshop and used Autodesk Inventor there. (But I didn't know how to use that either, and I decided that I would just go ahead.)
Anyway, I browsed the Google 3D Warehouse and downloaded a model of a Camaro. Then I scaled it from the full size down to the width I needed my wheelbase. (1.5") I deleted side view mirrors and other details (like the inside of the car).
I downloaded a free plugin that let me export STL from Sketchup. Then I pulled it into Partworks3D (on the Techshop computers) which let me do the CAM work. (That is the photo above.)