Depends how steep the climb is that you’re doing. W/kg matters more on a 10% gradient vs a 5% gradient both from a speed and gearing perspective. Like others have said, FTP w/kg doesn’t really matter since you’re gonna be out there all day- it’s about having the muscular endurance to keep going.
In regards to pacing, with proper fueling, you should be able to hold much more than 55-65% of FTP. I would think that your pacing would be similar to a full ironman, so ~75-80% FTP. You’ll have to adjust that for the length of your climb- a longer climb will have a lower % of FTP since you have less frequent recovery and longer intervals.
For context, I did an Everest at ~4w/kg in 12 hours total last summer (10:30 moving time). I did literally everything wrong- some jackass stole my cooler and most of my nutrition an hour in, the climb was steeper in parts than stava indicated (10% on strava which I could handle, vs actual 12-13% for .5 miles), and I had to ride to a gas station a few miles away a couple times because said nutrition was stolen. My strength is holding sub threshold for a long time, so even though I didn’t have a super high w/kg FTP, I should’ve been able to hold 85-90% of that FTP if I could’ve fueled and hydrated properly.