Circuit Breaker on Ammeter Bypass

This unit has a good rating for water/dust; it is IP67. So that part is good.

I would be rating the breaker for the expected load. If you are placing it so as to replace the fusible link, then it will not be in line with the new bypass direct from alternator to starter relay; this will be in the line from the starter relay to the inside of the car. So it will not be protecting the 8 guage wire; it will be protecting the wiring inside the car.

The biggest loads on that are inside the car in stock form are typically the headlights and blower motor, then blinkers and brake lights. That load ought to be in the 30-40 amp range, just computing off the top of my head. So, the breaker ought to be rated at no more than 40 A, IMO, and 30A if you want slightly better protection with some risk of false trips when you have the headlights and blower on for a long time. (I am open to other thoughts on this BTW.)

The reasons for this small rating are:
1) This is a thermal breaker; that means that it has to pass enough current for long anough time to get itself hot enough inside to trip. Flowing 200% of rated current (i.e., 60A through a 30A breaker), it will on average take 20 seconds for this to trip. You can burn up a lot of inside wires in that time.
2) The wires inside fed by the fusible link are smaller and can be melted/burned by the time this thing will trip if you have a partial short. In reality, the wires in the car are not large enough to withstand a partial short and survive anyway, so all you can protect against with anything in the fusible link location is hard shorts, that will make the breaker trip, or the fuse blow, very fast.

So, I would keep it on the smallish amperage side, for the sake of having some protection and risk some false trips... just IMO. You need to give an idea what the inside wiring loads you have are to decide this; for example, are you doign the headlight relay setup? Or is the headlight set-up stock?

And just don't think you are getting superior protection by replacing the fusible link with this; you aren't. It is all the same for a link or thermal breaker or fuse; there is an overload vs time that has to occur before it will break the circuit. Only a magnetic trip type of breaker will give you better protection. (And I can't tell you where to get one of those for auto use...)