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

Either one is good. Check into a good slider hitch if going with a short box. I chose the short box, and have a pullrite auto slide and like it a lot. My reason for the short box was partly in the trucks wheel base, figuring there would be better manuvering in camp grounds. I believe you get a better ride with the longer wheel base, but I have no complaints with mine.