Check out the finish in this ADCC contest between Jacare and Matt Lindland. Jacare works from north-south as much as any other top competitor. One of his preferred techniques from that position is the kimura. But the armbar is a nice option also.
It's worth remembering that, at one step in the process, the armbar attack from side control--both north and south side versions--have you going through the north-south armbar. I've had a big problem hitting all the pieces of the armbar attack from side control. I might have better luck if I break it down into (1) the transition from side control through scarf hold into north-south and (2) the armbar attack from north-south.
One key detail is that when in north-south, you want to underhook--rather than overhook--the arms. You could probably reach under and grab the gi under the arms and that way be ready to transition from north-south to taking-the-back. In any event, underhooking the arms from north-south will set you up perfectly for the armbar attack. If you are going to attack on your right side (right side underhook, the guy's left arm), then come up from the knee on the opposite site and then straighten that same leg out as you drop it across the guy's face. As you swing into the armbar on the other side, wedge your leg under the armpit of the arm you are attacking. That will probably be enough without having to put that leg across the chest, especially if the armlock is tight.
