Alternators-adjust my thinking

The whole alternator part number - alternator with pulley p/n - and amp rating is difficult to figure out.

TSB 66-78 is helpful even though its 1966
The 1970 Hamtramck Registry "1966 TSBs and Related Material Index" Page

3000010 is 60 amp alternator with 2.75" double groove pulley if installed as an ordered option.
If the same number is in the '68 FSM, then it was also the number used for the replacement.

37 or 46 amp would have been the alternators on v-8 Barracuda's in 68
https://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1968/68_Barracuda0020.jpg

Would expect those are easier to find.
The 60 amp alternator isn't even shown as an option for a-bodies, although the vehicle assembly drawings show they at least planned for that possibility.