According to calcs,
0 SpA EVs +6 SpAtk and SpDef Espeon Stored Power vs 252 HP/252 SpDef Blissey (+SpDef) : 114,29% - 134,45%
100% chance to OHKO
No EV investment in SpA is needed for Blissey.
For Chansey,
100 SpA EVs +6 SpAtk and SpDef Espeon Stored Power vs 252 HP/252 SpDef Eviolite Chansey (+SpDef) : 100,14% - 118,04%
100% chance to OHKO
100 SpA EVs is needed.
As requested on chat, with a Modest nature,
0 SpAtk EVs +6 SpAtk and SpDef Espeon (+SpAtk nature) Stored Power vs 252 HP/252 SpDef Eviolite Chansey (+SpDef nature) : 101,42% - 119,32%
100% chance to OHKO
So no SpAtk EVs needed.