This insert fits very well in the heatilator EC-36. It is the 1700, however I believe the 1200 (smaller) will fit is well. This is the biggest unit I could find to fit in the EC 36.
http://enviro-fire.com/fireplace-products/wood/fireplace-insert.html
I have the same model zero clearance fireplace, and did lots and lots of homework. The biggest issue is the flue outlet lining up (other than basic dimensions).
In other words, the stack for the heatilator is 6 inches at the center of the flue opening measured from the back of the INSIDE of the fireplace. A link to the manual for your heatilator with diagrams and measurements can be found here:
http://www.heatilator.com/customerCare/installManuals/index.asp
You need to go down a bit on the page and select your model in the box for "fireplace" . It is listed as "EC 36 (2 mb)" and is a pdf file. Look at page 6.
The kodiak center of flue is 6 inches from the outside of the back of the insert. So, you have only a one inch issue. You still have a little clearance as the kodiak is only 19 inches tall, and the collar for the flue on the kodiak insert is removable as well as can be installed from the inside of the insert firebox. See that same kodiak link for the insert install instructions with the dimensions on it.
Also, the inside of your heatilator flue pipe is 8 inches, so you have room to move the stainless liner in the 8 inch flue to line everything up.
So the SS liner can be fed down the chimney, the collar for the insert installed from inside the ZC firebox, then the insert slid in, reach up from the inside of the insert (the ceramic baffle and tubes are removable so you can do this) and pull the collar in place, then tighten the retaining bolt. Of course you need to use sealant, and of course a block of plate and insulation should be used, but I am just giving you a basic description, I am NOT telling you how to install this.
Edit: And you need a solid, good ondition ul approved ZC fireplace to start with as suggested earlier.
Hope this helps, KD