If you take (or bribe) Florence and then send a diplomat to the Papacy, you can offer them an alliance and give them Florence, and if you do it before the fifth turn, they ought to accept, just occasionally give them a gift to ensure the alliance holds, and you've got favour with the Vatican, not to mention that if you ask for military access (with, of course, a fitting price) you've got easy access to the Cornered Republic of Sicily. Have fun, hehe.