Used to own a '76 Club Cab with similar issue, turned out to be a connection issue where the ignition switch plugs into the main harness (midway down the column).

I found the issue by accident. Sitting with the truck one night waiting for it to warm up, I saw a very faint flash down low and the truck cut off. Closer inspection the next day, (if I remember correctly) the main power line and the start line were burnt at the connection.