I never done such a hillclimb...
My opinion to get power is: have a hilly ride for 2-3hours with your max. HR near to 4MM Lactat, but not above!
Then drive to a constant climb with aprox. 500-700m min. lengh (for race 5-7%, for mtb 7-10%), take the highest gear and start to climb from the stand as fast as you can for 30sec., slow down and wait until you HR will be in your "fat burning zone". Start again. Do that 10 times. Go down when you reach the top and start again until you have the 10 fullfilled.
Take a breake with low power, doing nothing was wrong.
Then start with the slow rpm-hillclimb like describt in the postings above.
The first part is to train your red muscle fibres, the second is for the white, which only works with oxigene (when my memory is right).
Try this in the last parts of your preperations and take this at the end of an
accending 3day intervall.
I will do that at the end of March, when I am back from my "training camp" and at least 4 weeks before my first race.
JUST FOR YOUR INFO: I am not a pro, but only interested in efficient training for my hobby -MTB Marathon races!