If your car will be street driven, there's another factor beyond the max output amps in selecting an alternator. It's the rpm range where that amperage is generated. Early alternators had to be spun up to higher rpms to reach max output. Modern alternators ramp up the amperage at much lower rpms. The driving scenario that forced the alternator change in my Cobra was stop and go traffic on a hot day after dark. Cooling fan and lights draw the power but engine is either at idle or if moving below 2000 rpms. So even though I had an alternator that tested as having 100 amp output, it wasn't keeping up with the demand because it was producing about 50 amps or so in this situation.
There's been about three generations of alternator technology since first introduced. For a street driven car, you want something that's at least 2nd generation, probably better if 3rd. In other words, an alternator that produces majority of amperage by about 2000 rpms or so. You can determine the max output needed by adding up the draw from major consuming parts (e.g. cooling fan, electric fuel pump, lights, etc.) and adding on a 20% safety margin, but use a modern alternator so it will actually produce those amps during real world driving conditions.