These wires are all labeled with a circuit number, wire gauge and color. "12 12red" would be circuit 12, 12 ga red wire. There is a table that tells you what the circuit numbers correspond to. If the color says "TR" that's with trace, IOW a stripe.
Tell me if I've got it.
The 18 ga green fusible link from the starter solenoid to splice B2 has fused. Splice B2 goes to the ignition switch via a 12 ga red wire. The 14 ga orange wire connected to the run side of the switch goes to the heater motor fuse. The heater motor circuit is a dead end here since it is protected by a fuse, and that would have protected the fusible link.
The #12 14 ga orange circuit branching before the heater fuse has to be the culprit. That connects to splice B6 which is shown on the Emissions page of the diagram. There are three devices fed by splice B6: the choke heater, the carb bowl vent, and two solenoids, one for the vapor recovery vent and one for the fast idle (anti-diesel?) solenoid. Your short is probably between B6 and these devices, since they all go directly to ground and would not have burnt up the wires.
I have heard of the electric choke wire shorting out before. That would do it. Note that the wires go from the ignition switch and through the firewall bulkhead connector and under the hood. Not sure what the bulkhead connector is like, but it's probably like the earlier models. I presume that symbols D4 and B6 with the heavy black box are bulkhead connections going through the firewall. If the wires under the hood to the emissions devices are not burnt, I would expect the short to be under the dash, before the wires go through the bulkhead connector.
Note that the short could have been intermittent, and moving the wires around could have fixed it. Best if you can find the short location by inspection. Any wire downstream of the short will be undamaged - you've got that principle?
Just me - I would first find the short and isolatre the burnt wires before doing anything at the fusible link. You can jumper the fusible link with a clip lead to test it; these are cheap and handy.
https://www.amazon.com/WGGE-WG-026-Piec ... 110&sr=8-3 They used to sell them at Radieux Shaque, but that's gone ... maybe a local electronics store, or from Amazon.