Ambitious rescue project!
Just a reminder that 1960 and newer Mopars, including the DeSoto were Unibody construction and the chassis plate JoHan used in the USA Oldies DeSoto, while somewhat basic in detail, is actually correct – unlike the earlier body-on-frame chassis JoHan used in the '60 annual kits for some strange reason. That said, the '58 Plymouth and '57 Chrysler ARE NOT an accurate swap into the '60 DeSoto. A more accurate, if problematic, swap would be the chassis from any of the Lindberg '64 Dodge kits since they reflect the Unibody construction of the era.