In any event, take a ohmeter and start working along the circuit and you'll find the answer.
If you haven't done this before. A cheap multi-meter will do. Disconnect the battery positive (so you don't accidently put voltage through the meter when its set for ohms). Clip or hold one probe to a good ground. Touch the other probe to remaining wire or into the connector so it touches the terminal. (If its the same as '67, it will be location Z
shown here.) A ground will show as low resistance. Disconnect the alternator's output terminal (Bat) and repeat the test. Then its a metter of what you find. For example.
-> Infinate resistance is normal. In which case, check the alternator's output terminal for to see if its internally grounded.
-> But if the ground is in the wiring. Then carefully disconnect the bulkhead connectors to see if the short is in the wire to the alternator or on the inside. If its inside, check the wire from Z to the ammeter, and the one from the ammeter to main splice, and mainsplice to P.
As you do this, remember everything after the fuse box, and the headlight switch, is protected by fuses and a circuit breaker. So no point in looking past those devices.
If it gets confusing, print that diagram or make your own to keep track.
edit IF it melted after the ignition switch was turned on, then it could be a short in the ignition or voltage regulation wiring. Just go through it methodicaly and you should find it.
