Greg L wrote
"That X through the centre is a vitally important part for the torsional stiffness.
However, it looks to be an I beam, a rather curious choice for a torsion member - it does actually work in bending, but if you are chasing torsional stiffness, and they weren't as you can see from the other details, you make everything out of box and tube."
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My well worn copy of Lincoln Electric's "Procedure handbook for ARC Welding" has a section on designing for arc welding.
The attachment is a few sections describing X members in some configurations being subjected to bending when the main frame is being twisted. I have not done any modeling to understand if a Studebaker frame-with-X-member's geometry falls in that category.
Dan T