Difference between 1967 and 1973 alternator on Small block?

So called round back alternators were made from the first ones in ?61? to 69, and then 70 when the same round back design CHANGED to isolated field (3 brush connections)--from 70 to about 72, when it was changed to the modern "square back" design that had the rectangular diode/ heat sink. These, generally, are a better design, and can be used on the older grounded field designs (pre 70) by simply grounding one brush and connect your green wire from the regulator to the remaining brush.

all of these will fit the same engine brackets

No sure about the pulleys, I don't remember a problem.