Hi, your horn should be the model B type ahooogah with a different shaped back end than a model A, sounds the same though, the wires pass throuugh a flexible stainless steel conduit in to the grille shell on the drivers side just below the headlight wires conduit, there are rubber grommets on the shell where the conduit joins it, the only holes in the firewall that are different would be those where the 4 cylinder would differ from the V-8, unlkess someone has drilled extras, and I am afraid I am not knowlageable enough on deuces to tell you, I do know an expert on them though, so if you are still perplexed, let me know, and I will put you in touch with him,
rolf@sasquatch.com Take care, Rolf