I had an XLT SP (97 model) and installed a 133 (X-12 SKS) rail and 136 x 1.25 track, and built a small rail extension to make it work. Do not go any longer than 136 on the X-12 or you will slam the track into the back of the tunnel. Concerning the X-10, Holz's initial rear skid bolted rignt in. Their torque arms and an X-10 136 rail. My recommedation is to leave the X-12 alone and ride the trails.
Try to work with the X-10. Bolt on 136 rails, take out the rear coupler blocks, and see how it goes. Try Holz and see if they have some older rear skids left. The Indy Lite skid is not coupled and will bolt in with some different torque arm to tunnel locations.
I would recommend against 121 to 136 rail extensions - a quite long moment arm working against the four mounting bolts.
Coupled suspensions are designed to thrust the machine forward with minimal weight transfer-minimal ski lift, just the opposite with the uncoupled suspension.
Lots of blather here, hope it makes some sense. Cheers.