Sounds like you will be approaching Stelvio from the north. In my oppinion this is the best way as the view is not so good on that side of the pass if you were heading in the opposite direction. Concentrate, look out for cyclist and worst of all, be aware of slow BMW motor bikes. Catch your breath at the top, take some pics, then blast down the south side, it is great fun.
According to your route you will be heading to Venice next. It is best to drop down to Lago di Garda, take another break and some pics, then blast long the motorway. Naturally you can't enter into venice, there are no roads, but you can find parking near the rail station and then walk or take one of the boats, it is well worth it.
The route back to Milan is quite boring if you go the fastest way so you may want to break your route, perhaps drop down to Parma for some cheese and ham before goinf on to Milan. Milan is not all it is cracked up to be but is a nice day trip. Piazza del Duomo is a must see and all of the main sights can be seen by walking as the city centre is very small. I live near to Duomo so if you need any advice let me know.
Sadly my Z is not here but I hope to bring it over some time.
Ciao e buon viaggio, Lee.