On my Dart I used a Napa alternator, it's a mopar "78 amp" unit from a Volare I think, here is a pic showing the part # and the RPM/output relationship. The RPMs there are alternator rpms. I don't recall the ratios, but I think once the engine reaches about 1700-1800 RPM (if memory serves), the alternator is spinning enough to churn out all 79 amps.

The mods I did to run this are:
* bypass the amp gauge
* 8 gauge wire from alternator to starter relay with a fusible link in it
* ground one of the outputs on the alternator to the tab on the back (not sure if that's the proper terminology, but there is a tab just sitting there for this purpose).

I point this out because it can be a cheaper alternative if you don't care about chrome, and don't mind an extra groove in the pulley.