I don’t think you necessarily need that much in terms of bunch riding skills, certainly some would be an advantage but not critical like road racing. It will be fast and furious on the first 12km into the bottom of the Glandon and you will need to be able to hold the front of the bunch assuming you arrive in the pen early. On the flatter valley sections you will most definitely need a group to post a competitive time but these bunches will tend to small and usually involve rotating up and overs rather than aggressive race bunch.
If you are with the first 100-300 into the Glandon then it will thin out pretty quickly and should find your pace with riders of similar ability. Important thing is you don’t overextend yourself to try stay with better climbers.
In terms of training you will need high levels of muscular endurance for the long climbs so plenty of sweetspot, threshold and V02max exactly as per plan builder. Again I don’t think its completely necessary to actually do these workouts on actual climbs as executing them on a turbo has been just as effective IMO. I am sure lots of rider here will attest to posting best times on climbs after months of indoor winter rides, certainly has been the case for me.
My advice would be follow the plans and try build up your muscular endurance, sustained power. Try get some prep events into the legs in April / May to get a feel for riding in a bunch. Any local sportives on the calendar? Take it you have no local cycling club?