there is a post in the "How to" section that does a good job of going through an xtra-10 swap, I'm sure you can follow the same guidelines for an xtra-12. The mounting holes might be different from an xtra-10, but they give a good explanation on how to find out where the holes should be. I'll be going through this next spring when I swap an extra-10 into an old indy 5, so let us know how it turns out! 
The X 10 swap I had to drill new holes. But love the results! You will have to drill new holes too. Not hard to do at all. But I think you may only have to drill new rear holes for an X 12. But like others said look it up or find someone with an X 12 from that factory and measure the mount holes...Good Luck its an easy job so don't worry
the extra will not be as easy as an extra 10 or edge swap. If your steering hoop area needs to be modded it is going to take alot more work. You will have to get dimensions of another extra 12 to mount it.
jdzaharia, Have you ever done this swap? Do you mind if I post your specs on my site? Since I have x10, edge and pro x skid mounting on there people are always asking for the specs. Also can you get a better picture of how the steering hoop mount endsabove the bolt?
A forum community dedicated to snowmobile owners and enthusiasts. Come join the discussion about performance, racing, troubleshooting, trails, maintenance, riding tips, modifications, classifieds, gear, and more!