I have a Delco 10si, 63 amp 9 o’clock  (AC-DELCO # 321-41, Lester #7127-9)    Tell the auto parts counter person that “The alternator is for a 1979 Buick Regal, 8 cylinder 4.9L (301W) engine, with air conditioning.”

I had to enlarge the upper hole in the alternator for the bolt that attaches the alternator to the head but the unit has worked very well and reliably

The selling point for a one wire alternator is that there is only one wire to hook up. Even those folks petrified of wiring understand that they can probably hook up one wire properly.

For street cars the three wire hookups are superior. The Delco 10si and 12si alternators and the Ford Gen3 alternators are very user friendly. With the three wire alts you can hook the voltage reference wire to the main power distribution block to make sure you are getting full voltage at all (o.k., most) users. Also, hooking up the excitation wire is simple on the early Mustangs.

I am using a Delco 10si 3-wire alternator as it provides the use of an "idiot" light (that is the quickest way to see if the alternator isn't working), and used the original Ford alternator mounts.  You'll need to enlarge the upper mounting hole as John Ha indicated  (from 3/8" to 7/16").  Note that the "9 o'clock" John Ha mentions can be changed by rotating the housing.

baker32r -  IMO, the advantage is the internal electronic regulator used in the later alternators, the reliability of these units is far superior to the older relay style externally mounted regulators .. again, my opinion.
Good reference on 1 & 3 wire alternators: http://www.madelectrical.com/