Welcome to Iboats,
Better call a Merc dealer and ask them, there used to be many high altitude jet kits for different elevations, usually in 1000 ft increments for each size. Bear in mind that if wanting better high altitude combo performance need at least one less pitch prop to pull wot revs higher and compensate some power loss due to altitude. Will work much better along a high altitude jet kit.
If can install an induction tach on tiller with new high altitude jet kit installed prior buying a less pitch prop will be better, that way will know how much wot rpm OB losses as currently loaded. Don't forget to reinstall original jets when returning to sea level boating...
Happy Boating
