Cad/Cam has killed some people... Gordon Smiley in 1982 at Indy as an example. My grandfather fixed the front suspensions on those cars after the CAD/CAM got it wrong. Garbage in/garbage out.
The USCartool uses the floor as a sheer plane to spread loads. Is it the best way, not really. But it does spread the load to a different plane with the welds being an area of concern.
If you ever see someone that wants to form a piece of sheet metal, they anneal the sheet with a oxy torch. The heat changes the hardness and chemical structure of the metal slightly to make it more malleable/workable. Less likely to crack when bent, etc. The same thing happens in the heat affected zone on a spot weld, that small area around the weld is more flexible and easier to work harden/crack. Nothing is the perfect solution, everything has a trade off.
