OK let's have a go...
my Swift which did the Alpine Classic a few weeks ago, has 451 wheels, a 42T small ring up front and I installed a 11-34T cassette for the event. I normally use a 11-28T cassette but for the 200km event the last 70km of climbing in high summer during the hottest part of the day, is very hard to do and I wanted looooow gears for when my legs want to give up, so I could just spin to the top.
Even that wasn't as low as I thought I wanted.
But the Alpine Classic while much longer does not include such steep sections, so my guess is even lower than 42T + 34T would be good to have. Not only that, but a bike setup that allows you to spin comfortably while on steep parts is also good to have. It would be no good if the front wheel lifted up, so perhaps push the saddle as for forward as it will go and tlited down, and put the bars quite far below the level of the saddle. This will be awkward to ride on level ground but uphill it should give you a good advantage.
So to recap, give yourself as low a gear as you can fit on th bike while retaining a good range. My range was 25-108"; I would go for a 39T and a 36T cassette on Mt Washington. Or alternatively train to stand on the pedals for very long stretches.