Losing body weight is the top tip for climbing.
Ride loads of road climbs sat down, but retain the ability to stand of for periods to give eveything a chance to stretch out and use different muscles. When out of the saddle change up a gear or two and back down as you sit down, this will help to maintain consistent speed as your cadence varies.
50 rpm seems quite slow, but that is a personal thing. I would use the perceived rate of exertion scale for training, and climbing Alpe D’Heuz for the first time is likely to feel extremely hard by the time you’ve done the first three ramps….
The main issue for me on the Alpe or anywhere else is the heat and humidity, not sure how you train for that here..