The haynes chassis design

[Talonmotorsport]

The thing with CAD is that it measures to 0.001mm where as steel box section comes with sides that are not perfectly flat,sqare or even uniformal and most people can only measure and cut to within 0.5mm. CAD does have it uses for 3D modeling but making it 3D in steel is another matter.
Fabricators will use real world methods for making things fit such as jigs ,squares diagonal measurements and common sense. Any body that has made their own chassis will have found at least 3-5 problems, mistakes,irregulaties with the design it just depends on how carefuly you measure and how critical you want to be with your build. People that have had to put packers in their front suspension brackets have found one problem but don't know why it happens, as others have found that the bars in the engine bay don't fit and theres the problem the the rear suspension to name but a few.