I'd be going for a 360mm personally. Anything less than a 350 (what I have) is a bit taxing physically in my opinion
I converted (aka butchered) a standard 380mm that I fitted to a standard Momo boss kit. The rivets on a standard earlier wheel that I ground off are the same pcd (70mm) as a Momo wheel, though when using a standard Momo boss, there is usually a bolt that sits at 12oclock. Doing it this way with an old Porsche wheel, you just turn the boss to have the holes sit at 10 to 2 if you get what I mean.
Long story short, the leverage of the 380mm wheel made it very easy to drive than a smaller wheel, as there is much less physical input when driving the twisties.
You could also have your Nardi recovered a bit thicker, though that's probably a $250-300 excercise. I'm not aware of any short boss kits, yet I'm sure someone could make you one if you were that keen