Crunch Doesn't get STAB off of seviper anyway.
So it'd do less damage then a poison jab.
My seviper has a Sp.atk of around 180 or so, and is a level 45.
When I emulated diamond I gameshark'd myself a team.
A level 50 toxicroak didn't do better than my under-leveled seviper, and still had a sp atk bigger by 10 than it.