You will definately need a multi-meter, with or without your buddies.
For simplicities sake, there's basically one wire to follow from the battery to the ignition switch. Its not physically all one wire, there are connections along the way, but that's all you have to trace to see where it burned.
There is a simplified diagram from my '67 at the bottom of this page http://www.heritech.org/cuda/Charge.html
Open your hood and see if you can follow it to the firewall connector. If the fusible link failed, it will literally be melted away. That's its job.

If it didn't then almost certainly something else was the weakest link and failed. Look between the firewall connector and the ignition switch. Thats a little harder for the switches in the steering column. But if you need the wire color, it will be in the '70 shop manual. Most likely black or red at least to the main junction.

Next you need your buddies with the multimeter. Because something drew too much power from the battery. Most likely a bare wire touched ground, and most likely one that is not protected by the fusebox.