My 1953 3100 under bed spare tire carrier cross channels are on top of the frame. frame pic The parts diaghram shows them under the frame top. Which is correct?
Mine were under when I took it off. I'd say as long as it doesn't interfere with the bed/its function, I'd say you're good where it is, but I'm no purist.
Mine were under the top of the frame but they interfered with the bumper brackets, tail lamp wire holes and the tire never fit snug. From the looks of your photo mounting them on top solves all those issues.