[litew8]

I did a quick search (on the net) and somebody comment this:

"A truck built as a chassis cab has a different frame than a pickup. A pickup bed won't fit on a chassis cab frame, and neither will 5th wheel rails."

And wikipedia says:

"A chassis cab or cab chassis is a body style, and type of vehicle construction, often found in medium duty truck commercial vehicles.

Instead of supplying the customer with a factory pre-assembled flatbed, cargo container, or other equipment - the customer is given the vehicle with just 'chassis' rails and a 'cab'. This allows the customer to assemble any desired aftermarket equipment; for instance, fire apparatus, an ambulance, or a recreational vehicle conversion package; which can be customised for the specific needs of the customer.

Aftermarket equippers, including coachbuilders, often make their own packages that can be ordered for the truck."

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... so I'm not sure, maybe that's why there isn't a bed on it already.
It might be worth more money to somebody doing a special restore of some of the above mentioned vehicles.
I'm sure there's someone more knowledgeable here that can help explain further.