Why do you need an external shunt? The stock ammeter has a built-in shunt. If you look at the wiring diagram for your truck you'll see that the ammeter does NOT measure the current output of your generator (or alternator). The loads are all paralleled off prior to the meter and the starting circuit bypasses it altogether. The only current the ammeter measures is the amount of discharge current flowing from the battery at any given time, or the amount of charging current flowing into the battery. The highest reading you'll ever see is if you crank your battery almost dead, in which case you either need a tune-up or a better battery. The only other way to see excessive current through the ammeter is to turn on every accessory you have (motor off), but why would you do that?
I have no idea why you would want a 100A main breaker on a truck. I worked at the telephone office right there in Gardnerville and the whole exchange that fed Topaz, Stateline, and Coleville too ran off a 400A breaker!
