Fusable Link

I don't know positively what wire size your 73 Duster has on it MopaJ but my 68 Cuda has 10 ga. wire running from the alt. to the bulkhead connector and the fusible link coming out going to the battery is 16 ga. I don't think you need to go any bigger than 12 ga. wire for your extra wire. A 12 ga. wire will divert 20 plus amps so that's a big load off the system. It don't matter if you run the wire straight to the battery or the relay since the wire running from the relay going to the battery is 10 ga.