Gavia and Mortirolo make a natural loop- we did it from Bormio.
Sormano is insanely steep and is now almost an art installation, with quotes and the altitude increases professionally painted on road – Google image it.
Spent 2 weeks riding Italian Dolomites and Alps and the only negative was the incessant close buzzing of loud piped Rossi wannabes (and I’m a lifelong biker with an R1 in the garage). We managed to ride Stelvio on Bike Day which was a free closed-road Sportif, but it’s in early September.
This guy’s site is an excellent resource.
Cycle the Alps Traffic-Free: Here’s How (2015 Edition)
And if you don’t already own a copy of Daniel Friebe’s Mountain High and Mountain Higher you should!