Smoke shelf confusion! Please help...

Hi all, I've gotten a lot of great woodburning info already from the forums, hoping y'all can help me with my fireplace conundrum. I have a block house built in 1948, about 780 square feet. It has a brick chimney with two flues, one running from the basement up, the other coming from a fireplace in the living room. Last winter my girlfriend and I installed an insulated liner in the basement flue running to a fisher papa bear stove. This heated the basement very well, but only got the top of the house to around 58-60 degrees, even after I cut registers into the floor. Consequently, now we want to install an insert in the main level fireplace. The problem is that the firebox necks down into about a 4" x 18" opening before entering the flue proper. I guess this is the "smoke shelf?" I am confused about how to approach lining this flue. Are there adapters available for situations like this? Do I need to get out the cold chisel and start hacking into the bricks?

Thanks!
Lucas

Cool, thanks for the input! There is no damper or metal in there at all, just some rotten looking masonry. I guess maybe I will get a sweep over for a little while to make sure nothing is structural?