Which is best for pulling a fifth wheel, short or long box?

Your choice. I use a short box with an autoslider hitch. I wouldn't chance using a short bed without an autoslide. Too easy to damage your truck. If you are going to be traveling a lot, I would probably just do the long bed. I like my short bed, but my next truck will probably be a long bed so I can get a hitch I can remove by myself. My autoslider takes 2 people to load or unload.