I did Alpine Loop last year and it’s very tough. My takeaways parallel what @llmonty stated. I was in the same w/kg range that you’re shooting for and was able to make the bell. FWIW here’s the link to my Alpine Loop file.
In terms of training plan SSB, Sustained Power Build, and Century will work. If I were to do it again, I would consider swapping the century plan for the Cross Country Marathon plan during the specialty phase. As you mentioned the climbs, while sustained, are not a steady grade but have varied pitches. While Having the ability to go into the red to cover the 9-12% pitches and settle back into 7-8% I think will help. Because let’s face it while climbing slow and steady, hovering under Threshold, is sage advice the reality is that we all probably either try and keep the same pace or stick with the strong riders in your group.
I noticed you said Specialty phase for you will end by late-July/early-August. If I’m following you correctly that would leave something like 6-8 weeks until the Alpine Loop in late-Sept. To fill that gap I would do one of two things: a) repeat half of the build plan and then follow with specialty or b) complete specialty as planned and then repeat half of it.
I’d like to come back but I don’t think I’ll make it this year. Our second child arrived earlier this Spring, so now it’s a struggle to just find time to ride. Maybe next year. Good luck on your fondos!