Modernized Engine Wiring

Bill,

Your voltage clamp using a diode to shunt the ammeter shunt is a good simple single component solution.

A second diode parallel in the reverse direction would also clamp voltage drop in reverse direction. However significant discharges without running alternator are often avoided to save the battery. It would be an extra expense without benefit.

Not sure of the diode used, there are high current Schottky diodes that have a lower forward voltage 0.46V at 100A. Some power diodes have forward voltages in the 1-2V range, too much to be helpful.

There are other solutions using circuity with power transistors or mosfets, however I like the simplicity and robustness of your design.