I put a 144x "2" onto my 99 670x; similar chassis, AFAIK.
The 144" came off my 03 Summit 700; it had 2" outer lugs, 1.75" in the center. I had to trim the 1/4" off the outer lugs to get it to fit, which it does, but barely.
It works a lot better now. I'm not sure how much different the non-X 670 is, but the only _real_ gripe I have with my 99x is bar height - too low.
Putting the 144" on helped bunches. Adding 2" of bar height helped a bit, but not enough (I'd go at least 4" if I were to do it again).
Overall, my 99 is a very capable sled. The only change I've made is the track (easy to do with a readily available extension, assuming yours has a 136" as it sits). It is super reliable, does what it's told, but I don't like the stock/stock+2" riding position. A longer track will certainly help; my 700 got TONS better in deep snow going from 144-->156, the 670 got a lot better 136-144. Not as dramatic as the 144-156" swap, but it did not _require_ a tunnel extension, either. The track hits the flap occasionally, though.
You'll probably get more/better answers in the Ski Doo section, FWIW. That said, assuming (who knows) the 99 and your 96 are similar, the only real change I wanted to make was a longer track & removing the swaybar; those two things made a huge difference.
Actually, the swaybar - I took it off my 700 first, and that helped. Finally took it off the 670, it made an _enormous_ difference on that sled for off-trail riding, with no real deterioration in trail riding (although I don't force the speed issue on groomed trails). If you plan on riding off trail, and if the 16yo is light, get rid of the swaybar, IMHO, again, assuming it is similar to my 99. I'm nowhere near "light" and it is Just Fine without it; and in soft snow, or sidehilling around in the trees, it is WORLDS better - with it, it tended to want to stay flat, when sidehilling, that's a PROBLEM. Plus, both of my sleds spend most of their time with two people on them; the absence of swaybar has not been an issue at all.
Iain
