The "66 Treaty" was clearly not something to be exposed. If the news of France and Austria joining hands to partition the European Continent was leaked, wouldn't the London Government go mad?
Maintaining balance on the European Continent is not such a balancing act. If Great France and the Great Shinra Empire were to emerge, the European Continent would truly become a triumvirate. At that time, it might be impossible to find a fourth country.
Like Pandora's box, once ambition is unleashed, it becomes difficult to put back.
Napoleon left the French with more than just glorious military achievements; he left them with a dream of a great nation. Stirred by their sentimentality, the fervor for expansion among the French populace would not be satisfied with just a good part of Italy and Belgium.
Adding Spain and Portugal wouldn't be excessive, would it? After all, Napoleon once occupied the Spanish region, and they must continue this fine tradition!