Shell Bell, like all health-draining moves such as Giga Drain or Oblivion Wing, can never get any credit for overkill damage. If the opponent only has 362 HP, you cannot drain for more than 45 with a Shell Bell Giga Impact, even if they have minimum defense.
If the opponent survives with a Focus Sash, then next turn they only have 1 HP left, and whatever move you use to finish them off will only let you heal 1 HP.