This is a little different approach, but if your not trying to be 100% original you can hook the horn power lead to just about any point that is an always hot connection. They may be calling the starter switch the starter solenoid. And the hot side would work fine for the horn lead. That is where it would connect on later years with a solenoid. That's where the wiring ends up anyway if you trace it out. Starter switch to ammeter, ammeter to light switch and light switch to horn. And all of these connections are alway hot.