Nick,
I tackled this same quandry a short time ago for my '67 GTV, which has the same size rims as your Duetto. Some AlfaBBers recommended that I go to 195/60R15's, while others recommended 185/65R15's.
I got a good deal on 195/60R15 Phantom Warrior (made by Michelin) at an America's Tire Co. store (
www.tires.com).
More about my GTV and it's new tires can be viewed here
http://www.alfabb.com/bb/forums/show...5&pagenumber=2. Included are comments by PSK that my tires might be too wide for the rims. However, I do not have any rubbing issues and have not experienced sidewall flex.