You'll need some type of relay so the horn button switch can trigger the horns. The only other purpose, besides a junction point for the main power wire (as you described) is to provide a buzzer when the key is in the ignition and driver's door is open.

Look at my schematic I drew. It's for a 69 Camaro, but will be the same for 68 and 69 F bodies. Colors might be different too.