Quick summary of the differences:
'70 up water pump is slightly longer than the '69 down, and the inlet is on the passenger side.
The alternator housings all mount interchangeably. The pulleys can be changed, but requires opening up the alternator to properly support pressing the new one on.
Since your electric system uses a voltage regulator with two wires to the alternator, you generally can't use a alternator with single field terminal.
Reason is that alternators made with single field terminals have the second terminal grounded to the housing and its brush is not insulated.
Save the early alternator for someone who wants an early style and do like Joe suggests.
