Modified Wood Stove

I have an old Dutchwest wood stove that I have been considering modifying into a type of Mass heater. Need opinion and advice before proceeding. I acquired a whole bunch of stone from a deconstructed fireplace wall. I am considering using it to build a type of "bell" on the top of my stove to create a mass for holding and slowly releasing the heat that would otherwise escape up the chimney, I have already removed the legs and built a base to accept the additional load, but have been delaying going any further because I'm not sure if it is a sound idea. here is the stove,

As you can see, I have stacked a few of the stones on the top to test it's heat holding capacity. But I intend to mortar it all together with refractory mortar, quite a bit taller, and encase the flue inside. Make it a few feet high inside the box of stone, similar to what you would do in the bell of a RMH barrel, then put a exit flue pipe down on the side going to the chimney, where the red circle is. Hope this adequately conveys my meaning. What are the thoughts and opinions of those more well versed in this type of thing? Is it feasible?

I forgot to mention, this stove has a catalytic converter built into the top. I removed it all as it had been completely degraded. Cleaned out all of the orifices and now it has more or less an empty cavity in the top that has an air injection port into it. I think that now acts as a secondary air inlet, as if burns very cleanly, no visible smoke when stoked up to temp. And the top temperature get pretty hot. It has a thermometer in the top and I have seen readings as high as 1500 deg. I appears that cavity is acting as a secondary burn chamber with the air inlet, and completely burning the flue gases. So there would be minimal soot buildup in the syster.

What climate?  House size/construction? Have you run it for an entire winter?  If so how did it do?  

Side note: Debbie Downer, your insurance agent, may not like the sound of "modified wood stove." One more thing to consider.