Yes you can use your existing alternator and add the different pulley. It is extremely unlikely your car used the 65 amp alternator. My guess is ether the 42 (orange ink stamp)or 55 amp (red ink stamp) alternator. The number of pulley sheaves does not typically relate to amperage . The number of sheaves are typically related to accessories.
Bob, thanks so much for your reply. I always appreciate when people much more knowledgeable than me can draw upon their experience to answer these types of questions.
May I ask why you believe mine was unlikely to be the 65 amp alternator? After my prior post, I spent quite a bit of time reviewing the generator pulley table and notes on pages B-10-11 in Mannel. If I'm reading it correctly, the only dual sheave pulleys for 1968 were C5AZ-10344-J and C7TZ-10344-B. But when I read the notes for C5AZ-10344-J, it says that it was used with "a 1965-68 55 amp Ford alternator,
except when AC is combined with either PS or TE."
My factory setup was AC + TE (but not PS), so that would exclude C5AZ-10344-J. That leaves only C7TZ-10344-B, which used the 65 amp model alternator. Am I not thinking about this correctly? I'm assuming Mannel is the bible when it comes to compatibility and what was used - is that a faulty assumption on my part? Thanks for any additional insights you can provide.