I've seen both combos in my travels. There is both aluminum and cast, but I haven't been able to ascertain what goes with what car/engine combo. My best bet has always been to restore the unit that came off of the vehicle. Just when I think that I have a year or application nailed down, Saginaw boxes tell me somethin' different.
PS - If anyone needs their manual steering boxes restored and rebuilt, jus' lemmie know.
Casey
734-439-8850
(My shameless plug.....)