Don't attempt welding on the cast iron manifold. If you haven't welded cast iron before, trust me you'd be better off buying a different manifold, because that one will likely be ruined. Drilling would be way easier, but you might need a longer than normal drill bit to do it. Cast iron drills pretty easily.
Interesting that Chevy saved a nickel by not drilling the manifold boss when they moved the horn to the radiator support.
Your firewall mounting bracket (assuming you'd fab it similar to the cardboard) would be pretty floppy. You'd need an angle brace to keep the horn from bouncing around.
