I did it and let me summarize my experience briefly. Maybe someone might find it useful later.

Carbonfiberboy's advice I think is the best starting point for everyone experimenting with HRMs. Stick to 90% and see what happens. I started at 90% and did Monitor's both side sticking with it. When I felt it was too slow I sped up a little.

On Ebbetts I started to dial in at the border of upper Zone 3 and bottom of Zone 4. That's around 95% for me. And it felt pretty much perfect. I was a little bit sad that I didn't push it on Monitor, but as soon as I got to Carson Pass I faded quite fast. Obviously my HR and speed dropped, so it might have been the best idea to go slower on Monitor. YMMV.

I second the idea of 34x30-32 for this ride. I went with 34x28 and my cadence was super low when I wanted to stick to Zone 3. Don't get me wrong. I can spin up these mountains, but this is not about speed, but finishing.

I did the ride with no cramps or major problems, so thanks everyone for advice!