Also remember that the factory gauge is NOT an "ammeter" - it's not in series with any loads; it's simply a sensitive voltmeter (galvanometer) that senses the voltage at the starter solenoid and at the horn relay screw terminal buss, compares them, and displays the difference as "amp numbers", to indicate whether current is flowing TO or FROM the battery. The last "ammeter" in a GM car was in the '62 Corvette.
The amp meter (really a volt meter) is reading the voltage drop across two points (which act as a shunt). I believe the two points are the horn relay and the terminal block by the battery. The wire between those points has a fixed resistance(acts as a shunt like all amp meters). As the current increases through that wire, a voltage drop is measured (IxR). The voltage drop is in proportion to the amplitude of current flowing between those two points. The meter is calibrated to represent amperage.