If you are mainly which EV'S to heighten I suggest going for...
252 on SDef
252 on Def
This is because his HP is already super low as seen here (This is Shuckle's Pokédex)
Alternately, his moveset can affect which EVs you use...
For example if your moveset had a defence boosting move, you could heighten his Def to make a Def MONSTER or heighten his SDef EV to balance things out, however you like it.
Also, who he is battling can effect this...
Lets say your Carbink (or Shuckle's as I've used before) was up against a Physical Sweeper. Now you'd want a high defence stat to counter it. Same with a Special Sweeper, you'd want the same thing with SDef.
So, reading that, what do you think now? It really depends.
Hope I Helped! =]