Did the Stelvio earlier this year from the Prato side. Was a savage beast, pretty relentless, but ramps up in the middle after like 10km of climbing, so be ready when your below sweet spot to have to raise the effort a bit. Also fuel well, no matter what training you do fueling is key.
Since joining trainerroad I wanted to build a base of power that will help with these long climbs, started out on Sustained Power Build which seems ideal. I really like the look of Century, but given I will be doing more triathlons come spring will probably toss a coin between that and 40kTT
Good luck though!! let us know how you get on.