Here's my experience. The first coversion I did to my 97 MM was to take the track from 136 to 151. I used a skid frame extension but had to move the skid toward the rear to make it work. I ended up with a 8" gap between the drivers and the tips of the skid frame rails. While it worked, I always felt the ride in chop was compromised because of the location of the skid frame. Later I installed a cat skid and the problem went away. IMO, remove your track, reinstall the axle and drivers and the skid frame with the extension on it and do the math on your new track length to be sure its going to work.
The tunnel extension I used was an MPI. can't remember if there was instructions about stiffening the sides of the extension but in preparation for a road trip I tied the tail down on a trailer and bent the extension. Bent it back and added home made stiffeners but it always had a crease in it. Later I changed the tunnel so no big deal. IMO, no matter which tunnel extension you use, pay close attention to the strength of the extension. The kit may come with enough stiffeners to make it survive but if it doesn't, be prepared to do what is necessary to keep it from bending, either from tying down, lifting or carrying a gas can or back pack. Hope this helps you.
