There is no question that the voltmeter is the gauge of choice as compared with the ammeter.
Some of the statements here on the HRBB about ammeters, however, are incorrect. Including Madelectrical.
While it is correct that some Manufactures (Chrysler) did route the full electrical load (current) through the ammeter, GM did not. Don't know about Ford. Who cares.
All the data I have seen indicate that GM used the main accessory wire as a shunt and measured the voltage drop (IR) across this wire to drive the ammeter. It's ohms law stuff.