67 had the horn relay under the hood, pin outs are quite different (I don't have the diagrams here, but several wires would still have to be moved). The horn wiring in 68-up runs forward with the headlight wires along the left side of the engine bay. In 67 the horn wires followed the path of the alternator wiring and jump to the right inner fenderwell from the alternator.
Running lights operate different, but I have not looked at the diagrams to see if that is where the differences are.
