Did this to 2 of my vipers with no problems, Been riding this way for 2 seasons and love it! You can look it up in the working log under "pair of vipers go 144".
Made my viper a 144 before the winter season. Put about 1,00 miles on this season and love it. I also put a boss seat on it too. Completely different ride over the 121. Can't stand riding my dads 02 vmax now. It feels like two completely different sleds now when they rode the same before. Check out super1c's build in the working log. A lot of helpful info from him!!!
Super1c and I both used rail extensions from tracks USA. The rail extensions would be the easier way to go. The 144 mountain skids bolt up different from the 121.