No Crank, No Lights, No Click, Battery Not Dead?

As mentioned before -check fusible link. Also make sure where the link connects that the snaps that hold the middle together is tight. wiggle the link and see if you get a connection. I would also look at the wires going thru the firewall there is one wire that that carries the main power to the ammeter and unless cleaned over the last 40 + years may have the infamous green corrosion buildup. Being a slant six you will have more access to get to this "Bulkhead disconnect". You would want to separate the plugs from the firewall and clean them out really good. Once clean, get some dielectric grease and squeeze some into each cavity to stop this from happening again. My final suggestion - Old man's short test. Remove the negative cable from the battery. Take a test light and see if you touch one lead to the battery and the other lead to the cable you just removed and see if the light goes on. If everything is off , doors shut etc. the light should not light!
I'm leaning towards the bulkhead disconnect! Good Luck