I only said I would stay with a 6 inch stove. The reasons are they are more efficient, larger cooktop, logs fit in lengthwise without rolling out loading sideways, and the burn characteristics are better.
Reducing from 8 to 6 is not a stove factor, it depends on chimney. If this is a straight up insulated chimney with top vent stove, adding a baffle and reducing may work well for you. If you need the full btu output, such as in a large pole building or barn where it is outdoor temperature, you may need the full capacity of the stove with the larger chimney that can carry the larger exhaust capacity.
The double door stoves were built for fire viewing. The first Fisher Stoves were the 3 single door stoves, and they were originally designed to set on a hearth using the original fireplace flue with a steel block off plate across the fireplace opening. People missed seeing the fire, so the wider stove with larger outlet was designed as a freestanding fireplace with door open and screen in place. With the word fireplace comes the loss of efficiency. If fire viewing is not a concern you are much better waiting for a single door or Bear Series.
For a legal installation in PA the stove needs to be UL Listed. That will be a Series III in the Fireplace Series, or Series VI in the Bear Series. They will have a UL tag on the rear shield, no angle iron corners, arched top door.
Reducing vent diameter is also against code in PA due to adoption of the International Family of Codes. The Mechanical Code is where you will find ALL appliances must be UL approved with label attached, and reduction of flue venting is prohibited.
People still install older stoves not knowing they are against code, or install and claim they were existing, since the code is for new installation, not prohibiting the use of existing. Many insurance companies will require a UL label as well.