Well, if you suspect the ignition switch, I'd start by disconnecting the battery, unplugging it & checking each wire going into the switch vs ground as you turn the key. There *shouldnt* be any grounds going through the ignition switch, but check the FSM to be sure. Then, check all the wires on the harness side of the connector vs ground.
How much leeway do you have with the FB? Would you be able to replace the fusible links with a more modern fusebox that would be easier to find?