RE: Cutting down 70 cm fin to 55 or 62 cm
You can cut off a 70 cm fin to what ever length you want,.......but why do you want such a short fin on such a wide board?
We've cut 80 cm back to 58 cm at test sessions, and a few minutes with a file and a piece of abrasive cloth and you should be back in business.
Often when you cut a fin down, and eliminate some of the taper or radius at the tip the fin seems to actually work better.
As WSGuy suggests, carbon fiber sanding and filing can be a bit hazardous to your health, so a dust mast is a very good idea.
But again, what do you expect to gain with a 55 or 62 cm fin span on an X-186?
Hope this helps,