what amperage alternator?

Generally the stockish 60-65 amp is fine as long as you are not adding a big stereo amp or other high load electrics.
There are 2 types also in case you didn't know.
There is the roundback and squareback style, and of the two the squareback is best because it puts out more at idle than the roundback.
Our 72 has the squareback which can be identified by it's square area for the Diodes, and a two wire field. (three total wires connected to it)

Also, since the AC cars generally used two belts the alternators usually come with a double belt pulley on them because they can be used for AC and non AC (1 or 2 belt)cars both.
This way they don't have to carry both.
The single pulley can be removed from a single and installed in place of a double though.