The cables are sized for the starter, which I believe dwarfs any accessories in amperage other than a winch. Do you need to run accessories while you are cranking the motor? Do the accessories power up from the positive terminal, the starter lug, or off of the main harness / fuse block? If you know your amperage, then you can use one of these engineering calculators on the web. Likely, this affects your decision for the negative cable / engine/chassis cable, and not the positive cable unless the accessories are wired from it.
I'm under the impression that the most vulnerable part of the system is the circuit between the alternator / ammeter / ignition switch / fuse block. On that circuit, a drop in voltage will reduce the voltage going into the coil, and you get a weaker spark, particularly while cranking the engine in cold weather. It would also be a wire(s) going on with a half century of oxidation if it is an original harness.