We don't have room for free-standing stove and we like the performance of this insert (already have same Fisher insert at our main home). Purchased this used insert from CRAIG'S LIST, cost a good bit to transport it from Westminster to Cumberland and being quoted too much for new one. Really need to figure out how to get this one to work. Just need to find someone willing to install. Flue should be big enough but being told there is a jog which could make it difficult or impossible. Also according to my measurements this stove should fit with 1/2" clearance so I am assuming company that cane to provide estimate just didn't want the job.
I agree 1/2 inch
depth is not a deal breaker.
If it extends out too far, anything like 1/2 inch cement board can be used as a filler behind face plate. It is only a cover for looks.
As far as the welded flue collar; The top crack is not the firebox, it is only the outer air chamber.
If a boot were bolted (or self tapping screws) to the top, the collar is not used. Normally the vent does not extend up beyond the outer air chamber.
As long as there are no cracks in the vent pipe or firebox top, I wouldn't worry about it.
Depending on the chimney height and other possible factors, a 6 inch insulated liner would probably work fine. It is not legal to reduce the size smaller than the appliance outlet, but it is also not legal to install any appliance without a UL Listing tag.
One reason for the larger 8 inch outlet was for open door burning with screen in place.
Another reason was since they were designed to use the larger existing fireplace flue, the larger outlet allows more heat up than a 6 inch would have. They were listed for using the existing fireplace flue, but time has proven the larger flues allowed too much expansion of flue gasses cooling them below condensing temperature, which can create rapid creosote formation. Hence the code change requiring the liner to be directly connected as well as the same size as appliance outlet. Connected to a 6 inch insulated liner, many work great.
Here's a thread with video on post #28;
https://www.hearth.com/talk/threads/cant-seem-to-figure-out.119184/page-2#post-1617042
I tested all the single door stoves with 6 inch outlets, as well as 8 inch double doors with the same 6 inch chimney when experimenting with the Smoke Shelf Baffle that was designed for the double door Fireplace Series. That chimney was in the interior, center of home, straight up with minimum connector pipe.
You will need a 6 inch liner if you go with a newer Insert, so I would certainly try it first, and if you find it doesn't work (which I highly doubt) you have a liner installed that is required for a newer stove.