There’s two ways to do it- the safe, legal, factory pinned fifth wheel plate, or the Billy Bob cobble-it-together method, of which there are hundreds of variations.
If you decide to modify yours, you are in luck with the Fontaine because the ones I have had were steel, not cast, and you could actually weld an ear on the plate. I’ll post a photo of one tomorrow. HOWEVER, the far better setup is the Holland factory pinned fifth wheel with removable pin that makes swapping between end dumps and regular trailers so much faster. That’s all I use now, if the voice of experience is worth anything to you. The downside is it’s going to set you back $1700 to $2500. But you won’t get a DOT violation with it, which you might with a Billy Bob job.