A reg cab Short wheel base cab-n-chassis truck has a 137" wheel base. Next longer is a 161"?
A Reg cab long bed Pickup has a 133" wheel base
A reg cab short bed Pickup has a 117" wheel base
If you put a dually bed on a cab-n-chassis truck, you will have an ADDITIONAL 4" gap between the bed and the cab.
So you can either cut the frame, driveshaft, fuel lines, etc and remove the 4", or make some sort of filler panel
But then you also run into the issue of the axle width. Cab-n-chassis have a NARROWER rear axle compared to a PICKUP dually. By appox 4". The fender WAY over hang the tires. I have seen a reg pickup bed put on and cut out flares used to cover the outer tires(think dodge mega cab duallys).
Also the frame rails on a cab-n-chassis are 34" wide. frame rails on a Pickup are 36" wide, so you will have to drill new holes in the bed to mount it, or make bracket on the outside of the frame to use the factory mounting bolt pattern.
Now for the "short box". Do you mean 8' or 6.75'?
I made my CCSB into f350 dually. Factory dually fenders will NOT bolt on to a single wheel bed. The dually bed is designed to accept the factory dually fenders. I bought conversion fenders and I also dissected a factory dually bed and grafted alot of the factory bracing into my short bed, made some of my own brackets and also modified the conversion dually fenders
My CCSB build