you mean a whole 1/2 hr has gone by and no one has suggested what an improvement a rocket mass heater would be, and how much money it could save you?.
others may pick up this thread who know exactly what a franklin stove insert looks like and what it's characteristics might be, if you are really attached to it and come hell or high water you want to adapt it to your specific fireplace.
It would seem however that you already understand some of the inefficiencies trying to heat this way, and compared to rocket mass heater technology even the best efficient woodstove setups are only about 20% as efficient as a rocket stove, so if you aren't familiar with that technology, there's lots of stuff on utube and paphlets and dvds and the like that would be good to look at before you pursue trying to hook up this woodstove just because you bought it already.
http://www.richsoil.com/rocket-stove-mass-heater.jsp before you invest lots more time installing and countless years feeding an inefficient system, at least look at this link to an introduction to rocket mass heaters
they are fun, creative, and every bit as charming as any old cast iron or steel stove (imho) plus use 1/5th to 1/10th the wood of a regular wood stove.
Think about it anyway, and if you do want to try and hook up the franklin insert it would be helpful to include pics of the fireplace and stove
welcome to Permies