Answering this because the answers below are wrong.
STAB boosts damage, not Base Power
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Type Effectiveness boosts damage, not Base Power.
Source
Actually the Aurora Beam Base Power with Never Melt Ice will be 78. (Or 71 if you're playing Gen 1, 2 or 3).
Also in this case you're fighting Rayquaza, every Aurora Beam user (except Legendaries) will not be a guaranteed OHKO to Rayquaza, you might be unlucky or your Aurora Beam user is below Rayquaza's level.
Lvl 70 0 SpA Walrein Aurora Beam vs. Lvl 70 0 HP / 0 SpD Rayquaza: 217-256 (95.5 - 112.7%) -- 75% chance to OHKO
Lvl 70 0 SpA Cloyster Aurora Beam vs. Lvl 70 0 HP / 0 SpD Rayquaza: 193-228 (85 - 100.4%) -- 6.3% chance to OHKO