From the pictures you posted.
Rear of engine oil leaks are probably valve cover/ half moon seals. But also look at the 2 big aluminum plugs. These will leak oil through he threads if not tight enough. I take them out and clean the threads in the head and on the plug put sealer on both and reinstall.
Picture with the red arrow I would say is the distributor o ring. This is often the most over looked source of an oil leak. They get hard then start weeping oil from underneath the dist. clamp.
Post a few more pictures taken directly of the sides and front of the engine.
I''ll take some more pix tomorrow. I think you're correct about the distributor hold down and there are definitely several areas around the head/block joint where oil is seeping. What do you guys do with these heads typically when you pull them? Check for straightness? Pull the valves and check em? I was thinking of having my machine shop do all that plus check the cam bearing clearances. Maybe the valve seals too? I was thinking of buying the complete engine gasket kit from classic Alfa and just changing all of them for good measure. I'll of course change those "O" rings you referred to when replacing the head gasket.
Thanks for the replies!