Chuck,
I have an NOS truck dual horn set that I mounted on my COE. Tim's picture is where they should be mounted. The instructions that came with my set show precise measurements where things are mounted. The wires that connect the horns to the relay are short and if you vary from the instructions they won't reach, so you have to follow the directions carefully.
Your horns look different than GM horns. For example, the horn wire comes out a relief in the cover and does not have the contacts as shown in your picture.
Unless you are looking for a factory correct restoration, I would argue those horns are part of your truck's history and you should see if they work, or fix them, and use 'em!
Kent
