Reagan Democrats were leftover conservatives in the Democrat party. If Giulliani runs what would Democrats vote for him for while they can just vote for a person who holds his social positions in their own party?
You have to have some social appeal, but not too much ;)
Cross-over Democrats wouldn't be voting for Guiliani because of his social views, they would be voting for views on issues like the WOT.
Actually, the big voting block that Guiliani appeals to are the independents with Libertarian leanings. As a group, they tend to be moderately conservative on most issues except social issues. This is the block of voters who are scared by the Religious Right (people like myself and presumably, you).