The golden rule is the fit a protection device to safe guard the rating of the wiring.
So, if the stock alternator output was, for example, a 40A unit then the existing wiring should be capable of handling that 40A, so it should be protected with a 40A device.
This is the inherent danger of wacking in a high output alternator and connecting it to existing wiring not capable of handling what the alternator is capable of producing
Can you spell "FIRE"
I recommend to insert an inline maxi fuse [easily maintained] or fusible link [not so easily maintained] to protect the wiring and fit that device at the Alternator "A" terminal, in series with the wire[s] that go to the fuse block[s].
My recommendation for additional wiring which dictated the upgrade in the first place, is this: -
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This auxiliary power feed system can be quite simply overlaid on the existing wiring but will protect that wiring from the new current loads.
You can use the latest technology wires, relays and protection devices knowing that the wiring is dimensioned correctly and protected adequately.
It is the LOAD that consumes the current from the Alternator, the battery only wants a trickle current to keep it charged up so it can crank the engine next time you want it to.
In normal circumstance that is usually the current resulting from a 14V alternator voltage feeding into a 12V battery so its only a 2V P.D. and a few amps, depending on the battery condition.
The gauge cable from the battery to the starter needs to be adequate but only for the characteristics of the starter motor used.
The engine earth [Ground] cable must be the same gauge since it will handle the same current back to the battery as the supply cable does to the starter.
Upgrades to these two cables would only be necessary if the original cables were under dimensioned and you experience slow or erratic cranking.
I like an Ammeter in the circuit so I can see what is going on
Clear as mud