Northstar or Constitution, are these too much heat for my home ?

My second post what a great forum, I have been reviewing posts here for about a month and have decided on 2 different units. I am removing my Preway FP for a more efficient epa rated FP. I am pretty much sold on the Heatnglo Northstar or Heatilator Constitution. my biggest concern is these may be to much heat for my space. The main living/dining room where the FP is located is 20x28 with 8' ceiling's. I have a 1600 sq ft bungalow roughly 40x40, obviously the heat will seep into the kitchen and other bedrooms on the main floor and the front door foyer/ upstairs stairway is located off of the living room (this area we close off in the winter with the french doors I installed to help keep the heat in the living room and only heat the upstairs at night via a separate thermostat / zone, it's the master bedroom). I am on a slab with a nat gas boiler / baseboard water so there are no ducts in the house.
If I were to install an aux air kit the only way I can see to run a duct would be up through the external chase to tap into the upstairs master bed room (if that is even a possibility). My preway is flush mounted into the chase.
With all of that said I mainly want to heat the downstairs and don't want to be roasted out of the house. Some threads say install a smaller unit and run hot, other treads say go big for a longer burn time I am confused. I am in Michigan north of Detroit and it does get cold but my house is pretty tight. Is there a smaller unit that is 75% efficient that will apply for the tax credit (I really want the tax credit and to be more efficient), or should I go with the Northstar or the Constitution and are these to much heat for my home ?
Thanks in advance for any advice, Mark

Thanks for the replies, By aux air kit, I was talking about a blower to another room. I was hopeing to pull some heat into the upstairs , or maybe a better option would be to possibly a return duct vent from the upstairs (with out another blower) to the FP which would draw heat from the living room up. Has anyone done this with a return duct through an external chase, that would be my only option for a duct. Or I can just open the doors and use a fan to blow heat up.
As for how often, I will be burning most of the time, if I can achieve a long enough burn durring the day (I doubt I will get an 8-10 hour burn time from when I leave for work to when I return), right now I am burning when I get home until 11 pm and I shut the blower off on the Preway with a slight fire and lots of hot coals.
Thats it durring the week, on the weekends I burn 6 am to 11 pm, I don't trust leaving a fire going in that old Preway unatended. With a good unit and an air tight door it would be safer to run all night and reload in the am.
I guess I need a bigger box for longer burn times.
Thanks, Mark