They're damn near IMPOSSIBBLE to fully remove. Before you butcher it up too bad, I'd recommend getting a die grinder (90 degree ones work great) or a dremel tool and just cutting the cage off flush with the box; It's not worth trying to muscle the damn thing out. (I broke the little pin off of my inlet air temp sensor when I tried)
After that, the gasket goo they give you will fill up any irregularities you've created. (I just made a gasket out of some sheet rubber we use around the shop for making gaskets)