I think the headroom was only 5,10" inside the box. In order to make the overhead go over the cab, the box floor and frame would have been 6+ inches above the frame. The wheel wells wouldn't line up.Etc. By cutting and using the top I was able to get about 6'2" headroom.
From the beginning I wanted something similar to a Callen Camper shell. If I were to do it over again I would build my own and make the cabover part big enough for a bed. By not having the bed above the cab it takes up too much space and limits the layout of the lower portion of the box.
it was nice to have a king size bed , but even folded up it took up too much space.
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