An arm triangle is fine as a transition when he reaches for the head. If he gets to the knees, practice transitioning to the darse, that will help a lot.
A lot of times when I do the North/South choke my opponent simply grabs my head and rools onto his knees (we get to a mutual schoolyard headlock position). Should I practice more tightness in the choke or should I try to learn the variation with the arm in?
Really, though, I suggest that you keep your balance. Once you slip the arm around the head, creating the choke, start to take your weight off of their body to finish the choke.
If he starts to roll up when you have fed the arm around his head, just drop your head and chest into his chest to keep him flat on his back.
Basically my point is, if he starts to roll up, you're not keeping your weight down enough, and you need to smash him.