I did this recently with my Sig 556. Rifle vise (regular vise and my wife's best hand towel), and the wrench you have shown... um, and a hammer... as the Sig peens their keepers.... Ruined the towel and the wrench, but I got the MFSOB off.
So tell us what stock you put on? I hate the factory stock on my 556 and have been considering a replacement.