Hello, I think my first question would be what is your experience like in the Mountains? You mention that you haven’t done a climb with this level of climbing before. If you haven’t cycled in ‘real’ mountains before I highly recommend (if it is possible), having a practice run. A couple of days where you ride some serious elevation will give you the best idea, not just of pacing, but also critically of altitude, hydration and fueling. It will also give you confidence if you are able to cover say 4000m in training. It is hard to replicate the constant pedaling required on long climbs in high mountains anywhere else. You can obviously do hill reps, buy a wahoo climbr, put your front wheel up on a block whilst using a turbo. General guide is get your FTP as high as possible, be as light as possible (both man and machine) - get some lightweight climbing wheels.
Beyond this, if you have no power meter, then HR and VAM will be the next best option. Remember the HR can go really AWOL in the first couple days in the mountains. Unless you are seriously gifted, then the answer to your pacing will be ‘slow’. The reality of 7% average for 90 minutes is some very low cadence, so I would advise a 32 on the rear (don’t be shy - Froomey uses one)., this could seriously save you at the end of the day.
Finally - on the day - don’t get overexcited - a conservative pacing strategy with a negative split, so you are passing others in the final 50km, rather than being passed will feel so much better and probably give you your best time.