I’d stick with the 40k plan.
Reason being, that climb would take me (3.8w/kg on a TT bike) about 17-19 minutes. That length of interval I’m only going to be able to go a little bit over threshold, 105% at most. And that isn’t even taking into account the other 34km you’ve got to ride.
Maybe substitute a couple of sessions for outdoor practice close to the event, to get used to climbing that kind of gradient on your TT bike in aero position? Assuming you’re doing it on a TT rig, of course.