Although this dates back to 1952, the methods were the same that I encountered in the 1960s, and I thought some of you might like to see this.
www.youtube.com/watch?v=FzzGCF4otss
The same principles applied to designing and building trucks (including the clay modelling and the mock-up interior bucks). Things like gearshift linkages were developed using these full-sized models.
Later on I was involved in the physical testing of the complete vehicle before it was released for production. Exactly as shown in the film.