I think you should add in fetchlands, 4-6, for deck thinning.  As you have no card draw(other then Necropotence, which is dangerous w/Pox) you need a way to get more from your deck.

I like the suggestion for Chains of Mephistopheles.  I run two in my maindeck, as so many decks have massive card advantage engines, its fun to shut them down once in a while.

I'm a fan of using Crucible of Worlds in Pox decks.  Recurring Wastes and sac'd lands is good times, as well as playing Entomb for Strip Mine to start the land lock.  Also, with five reuseable Strip effects, you can ditch the less the optimal Sinkholes.  

The mention of E.Plague is solid.  Not only will it rid you of pesky welders, but it also takes care of those little tokens that Oath decks try to give you.  Definately worth a look in the sideboard.

Hope I was of some help. Good luck!