that connects the gooseneck to the sliding mast car(you can just see it below the pin you mentioned, Max. I say 5/8" plus because it may be metric(5/8" drill bit seems sloppy). I think that would be the place to attach a horn on each side. I'll have to get a longer bolt to accommodate. Rig rite has a large hook(thanks) that might work. On the other hand, it might be cheaper to have them build a couple down the hill out of scrap bronze($ will win).