Study these pictures from Brad Allen. They helped me even though they were for a earlier truck. Brad's pictures are for a short box, but I would expect that you should get an idea how it all works. Note how the holes in the angle strips line up with the cross sills. So, if the angle strips are welded on your sides, that should help you locate the cross sills.

https://www.flickr.com/photos/advance-design_parts_co/sets/72157631884205225


Gord 🇨🇦
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1954 1/2 ton 235 4 speed