Try not to get trained too much to block low at certain times and you won't find yourself going "Oh no he's doing 6B!! I need to block high!" two frames before you get hit. Just focused and don't go on autopilot, and V./Sion 6B and W/Arc 5[ B ] won't be that hard to block on reaction (other examples: Shiki/Nanaya 5B[ B ], V.Akiha 6C, Hisui assist)
As Tropsy said, the REAL fast overheads (especially instant Kohaku j.C) you really just have to guess. I swear, that Kohaku j.C....it's murder.