When I split the cases on my 79, I ram into the same problem. The alternator needed to be removed. You may need the special tool honda recommends to get it off. Unless you've done so already, the oil pump neda to come off as well.
Good luck, and keep us informed.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.