Answering this because the answers below are wrong.
STAB boosts damage, not Base Power
Type Effectiveness boosts damage, not Base Power.
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