You wrote: "Which side rail is the best / most stable for this set up?"
If I am understanding you correctly, you are saying that the rifle does not have a side rail and you want to mount one. If so, this site can help you do so: http://www.gunsgutsandgod.com/railinstall.htm
This site, http://www.tantal.kalashnikov.guns.ru/bstmounts.html
, will give you a selection of side rail mounts and mounting hardware to choose from.
Below are a couple of pics of my EKP-1S-03M Kobra mounted on the side rail of my SAR-1. The set up is very stable and the sight keeps it zero if it has to be dismounted and then remounted.
Here are the instructions on how to zero the sight: http://tantal.kalashnikov.guns.ru/kobrazer.html
As Tom said, you will not be able to use the iron sights when the Kobra is mounted. Co-witnessing is possible on some of the newer Kobras but not on your model.