Ted's advice is good. I have converted one jacket myself:
http://www.xmarksthescot.com/forum/c...arlander+crail
From those pictures, you can see that the top of the kilt is not an issue (note that the kilt in the pictures is actually a sports kilt, and being worn low). What is more important, is that the cut away in the front has room for the sporran.

Another thing to note is that you do not really have all that much control over how long the jacket ends up. You trim the jacket so that just one button is removed. It turns out that this mostly dictates the new length of the jacket.

Enjoy your new kilt jacket!