Size 50 De Rosa should fit almost exactly the same as your Bianchi with a 100 mm stem and with 15-20 mm of spacers removed, assuming you are using a -6 degree stem and more than 20 mm of spacers on the Bianchi (conical top cap included). Alternatively you may need to use something like a -17 degree stem on the De Rosa, which would actually look quite good with the flat top tube of that frame.