I did it differently since I don't care about how original it looks.
I figure 30 amp is good since the original alt was 37 amps and powered the whole car.
I think there is more in the passenger compartment power draw wise than in the engine compartment.
If you relayed the headlights, then my guess is about even.

I fused it instead of a breaker, but that would work too. I also fused in incase I needed room to grow. Power windows and what not. Then I just needed to buy a 5 dollar pack of the higher amp fuses, or feed it off another line.