Ron, I'm gonna mess up your plans. About the only rails that the KX3 fits inside of is the MI stuff, and some YHM stuff. EDIT - There are other rails out there - can't recall them off the top of my head. The KX3 is 1.375" in diameter, so it'll need to be something over that as an inner diameter...
Now, the KX5 is a different story... I'm just sayin'... Did you look at the KX5 yet?
EDIT - Pics of the KX3 inside the MI SS-10. 7.5" barrel, pistol gas, KX3. The MI SS-10 fully covered the KX3 but about a 1/4" inch, so I chopped the SS-10 back a little. Saved as much rail on top as I could, to kick the front sight as far forward as I could. I cut the rail back to a shade over 8.5".