Need an Adult. Alternator Wiring Question. Non-OEM System

Understood. Since I've got a 2AWG wire running from the rear battery to the firewall bulkhead, would a 125amp breaker be sufficient?

The distance is only 2.5ft from the alternator to the firewall. From the firewall rearward, it's the big main wire.

Just not sure what could handle that load with that large of a wire. Especially during starting.