I vote vertical but on a longer chain. Spouse has a similar brooch conversion that is narrower and a little (but not a lot) shorter and a jeweler kept the pin element intact and just (invisibly) added a tiny bale on one end.
If horizontal, I would mock it up with thread and use the uppermost vertices (for mouse ears) and not the furthest E and W points to keep it from flipping. It may look too severe -- hoping to avoid an "NFL goalpost" look with almost right angles where the chain meets the brooch.
Also: I don't entirely know what I'm talking about here -- just trying to mentally visualize it.