Post a pic when you can....but as an interim....
Place a small gauge jumper wire with an inline fuse built in, between the Positive battery post and the junction block, and try the headlights....if the fuse blows, the headlight circuit has a short somewhere...
Remove one end of the fusible link and using an multi meter set to Ohms, measure the resistance of the fusible link....If it is open circuit, it has burned up due to a short somewhere...
To me it sounds like the fusible link melted causing a loss of supply voltage to not only the headlights but also the Ignition switch circuits....
These old trucks are a basic system that has several fail points,,,,bad grounds, corroded connections at connection plugs, especially firewall to cab connectors and failed fusible links...plus the occasional bad component like an Ignition switch or solenoid...
I would concentrate on one item at a time...with first item to test being the fusible link...
If it fails you will have a truck with nothing that works...
