I second what Donna has said - the motorway route is actually a better road to travel on, since they are wider and have more lanes. If you stay on the right lane, the faster cars pass on the left lane and you won't be bothered.
I would follow the route that Google Maps suggests - the Firenze-Siena to the A1 in Florence, then up to Modena and from there toward Verona and Lake Garda.
Italian drivers are not dreadful - they are aggressive. On smaller roads, they tailgate if you go slow but pass as soon as they are able. Even on those smaller roads, just drive as you feel you can safely do so, and forget about any cars behind you.
It is just their way of doing, even when you are going at a decent speed they often times are impatient and want to go faster. In Tuscany, there are lots of foreign drivers so you'll be in good company ;-).