Installing direct vent gas fireplace inside removed firebox but keeping exterior chimney

[leem]

After a few frustrating years of searching for a gas insert to fit inside our existing masonry box, we made the decision to go with a direct vent gas fireplace with the intention of removing the chimney and framing for the new unit. We had to replace our roof, so we removed the chimney to just below the 1.5' soffit. As we have been searching for the right unit, it started to occur to us that we might not have to remove the entire chimney, just remove the interior bricks (possibly to accommodate a deeper unit) and cut a hole in the chimney to accommodate the vent.
I haven't found any discussions like this online, so I came to this site for some ideas. Thank you in advance. Lee