Hey NewGuy,
I would just have a local installer or sweep come by and take a look. My fireplace is built out of fieldstone and at some point, someone had put in a heatilator (metal) firebox. I didnt know much about it and when I had someone come out and take a look, they said it was pretty far gone - very rusted, etc. They suggested getting an insert or a freestanding stove and getting rid of the metal box if necessary.
I followed their suggestion and basically cut out the back and top of the metal firebox. This opened up a very rather large area, constructed of stone, which allowed me to easily run my liner SS liner up through the clay lined chimney.
I suppose it’s what is behind your firebox that is important - if it’s framing, you’re probably out of luck.. If it’s masonry, you might be in good shape. If it’s sold as a freestanding as well, and you’ve got masonry behind the metal box, I don’t see how that would be different than installing it freestanding, with the required surrounds. Best to check with an expert , though!
Good luck,
Scott