Try Friel HR zones? Those map better for me versus Coggan HR zones. When I first started riding 5 years ago, I quickly figured out that my all-day HR was roughly 135-140bpm and thats how I rode the DeathRide. IMHO you can only determine that by going out and doing long (5-8 hour) rides, preferably in the mountains but you can do that on the flats too. My only other suggestion is to figure out your climbing cadence using FulGaz and if its lower than your normal cadence (due to gearing), then do some training rides at that lower cadence.
Again you got this! You’ve done long rides with similar climbing… Simply go into the event with fitness fully baked a couple weeks ahead (end of June), carb loaded, and a positive mental attitude 
Climbing up Monitor West with 2000+ like minded folks and then descending Monitor East for an epic ~17 minute freewheeling cruise at dawn 
should inspire you for the rest of the day!