Hey, Mike!
I'm restoring a 1937 1-1/2 and keeping the truck as original as possible as well.
On my truck with the original 216 CI 6 cylinder still in it, the horn mounts on the top of the intake manifold forward of the carb, just as Tiny indicated.
On the headliner, I replaced my original tattered cardboard headliner with one I purchased from Jim Carter Antique truck parts. It is a one piece unit that you form with an ammonia/water solution. It was a real booger to get it to fit and in retrospect, I might resort to a fiberglass reproduction that they offer for aesthetic purposes.
