Anyone with experience in manufacturing stamped steel knew the issues with completely flat panels, as well as dealing with stainless steel.
Ford made a dozen or so stainless steel Fordor sedans as far back as 1936 as part of an R&D effort in materials science. They learned a lot. The material was difficult to work with, costly and difficult to repair. The cost of the car made it uneconomic.
They repeated the exercise with a Lincoln in the 1960s and with the '58 Thunderbird, more for publicity than anything else.