I have very extensive experience with the rain tank set so I can help you there.
There needs to be enough SpA EVs to OHKO Keldeo with Hurricane. That is 124 EVs in SpA. 76 EVs is really all you need in Special Defense. It allows you to never be 2HKOed by Tornadus-T's Hidden Power Ice at full health (it does survive Genesect's Ice Beam). HP can stay the same. 56 EVs remain, and placing 48 in Defense gives Dragonite enough to survive CB Nite's Outrage. The remaining 8 can go in Attack to slightly pump up Dragon Tail.
EDIT: That gives 252 HP / 8 Atk / 48 Def / 124 SpA / 76 SpD and a Calm nature
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