Here is the stock diagram. I have bypassed the horn relay because the trucks from 67 up moved it under the dash in the cab. In reality as long as it connects to the positive battery terminal it will charge the battery. Wiring it like the diagram will give better power to the accessories.
Here it is in real time.
The wiring charts for AWG 12 show the amperage carrying limits for a given length of wire and for most applications it is enough. 10 gauge will give a smaller resistance and 8 less yet.