Full current amp gauges were fairly common in older vehicles, but the output of alternators was also usually quite low (about 40 amps or less). As alternator amperage increased, the in-dash amp gauges were changed to a shunt-type amp gauge, which does not flow the full current through the wiring.
An amp gauge that uses a shunt under the hood (used by GM in the 70's) works more like a milli-voltmeter that measures the voltage drop between two points in the wiring harness. The gauge then uses the measurement of voltage drop to calculate an amp (or current flow) reading on the gauge. Its similar to measuring the amount of fuel in your gas tank at two points in time. If you know the amount of time and the fuel level difference between the two measurements, you can calculate the fuel flow rate.
The article referenced in a previous post by Rick 5150 69 identifies a replacement cluster that uses a voltmeter instead of an ammeter. This is a much safer choice.