I would assume that swaping an AT to a manual would be at least as expensive, and probably a little more than replacing/rebuilding a damaged MT. With labor, putting a rebuilt MT to replace another one will cost $800-$1200 in parts, and at least $300-$600 in labor.
I think the AT would cost more in parts, not to mention there would be some other things they would have to change, like the console and what not. If you go into a shop for that, you definatly will find some more problems that need to be fixed as well, so have extra cash ready. =)
By the way, this is our Montreal, It's a 1972' Alfa Romeo in excelent condition (less than 30,000 miles), and is one of less than 200 Montreals of any year that are in the united states and actualy run.
http://kenohki2112.homestead.com/fil..._side_view.jpg