My observation may not be helpful, but here goes.
I recently bought a 1961 parts truck with the x-frame. Someone tried to fit a 1965-72 12 bolt rear and trailing arms onto the x-frame. It’s very clear to me that the geometry is wrong. The trailing arms contact the X part of the frame with any weight on the frame.
I suspect that the mounting brackets on the rear axle are closer to the centerline in the 1963-1972 geometry, but I am not sure. Perhaps the brackets that mount the trailing arms to the frame or closer together for an x-frame chassis.
I do know this, the trailing arms for a long bed are the same as the trailing arms for a short bed.
