A second vote for Alfabb.com. Go to "forums" at the homepage, and see the subsection called "GT (1965-1974)".
Because the Alfabb covers all models of Alfas (unlike the E9 forum which just covers CS coupes), its organization is a little more complicated. Someone with a question about the carburetors on their Alfa GT might post in the "Carburetors & Fuel Injection" section, rather than in the GT section. So, you need to noodle around a bit to find what you are looking for.
Use the search feature before posting a question - the AlfaBB has been around for awhile, and most questions have already been answered at least once.
I can't think of a second source of online information about Alfa Romeos - the AlfaBB is pretty much the only game in town.