I saw this question over on the CLC forum and thought reposting and consolidating my findings here on our site. I had posted this as a reply before but will repost and tag it and save to our Help Page. Note, I am not trying to debate which is better just stating facts of what came on the cars from the factory. Here is the info I had posted:
Literature descriptions of the New for 1963 390 engine, the Serviceman Bulletin, and the 1963 & 1964 shop manuals, all refer to an aluminum water pump. My best guess is at some point they switched to the cast iron casting (which some prefer), and perhaps alternated between aluminum and cast iron. Just to note, I have personally never had any issues with the aluminum water pumps. I've had an aluminum one on mine since I bought it since 1996 and remember maybe having to replace it once 10+ years ago.
For reference if you look at the 1962 October Serviceman bulletin page 48 it says "A new die-cast aluminum water pump, which is serviced as an assembly only". There is a scan on the 63/64 site at this link: http://6364cadillac.ning.com/photo/1962-pg-48-oct?context=album&...
Click 'View Full Size' and the + in the magnifier to view full size.
The 1963 shop manual refers to a 'die cast aluminum water pump on page 11-1.
The 1964 shop manual refers to a 'die cast aluminum water pump on page 7-1.
I have also seen it listed in other early literature such as the Dealer Data Books (see page 88A in the 1963 Dealer data book), so am pretty sure that from the factory they were aluminum.