Andy's right about the plug, but if you are melting wires then you are pulling too much current through them. If you have relays and the ignition switch wires are actuating the relays, you should not be pulling very much current through them.
The large red wire is the ignition feed wire and the pink wire feeds the ignition coil, the fuse panel, and the gauge cluster.It feeds the key on circuits and the large red wire feeds the fuse panel for the constant on circuits, hazards, brakes, front parking lights, taillights, dome lights and cigarette lighter. It also feeds the headlight switch.
The large brown wire on the key is the accessory feed and it powers the rest of the fuse panel, wipers, turn signals, and the heater switch.
You should have a main wiring junction in the engine compartment where the battery, alternator, cab feed wire, and the cooling fans tie together. You should feed the PCM relay, the fuel pump relay, and the cooling fan(s)relay from here.
Then the key switch pink wire will control these relays whe the key is on.
Here is a complete diagram on the 67 to 72 trucks stock from the factory.
It's the second post in the thread.
http://67-72chevytrucks.com/vboard/s...185856&page=13