Second The 400mm Question
That screams to me that you need a substantially bigger frame. Although if you have it already not much you can do there.
Also the Kalloy will deffinately be 'safer' than a Thomson, RaceFace, Etc... as it is almost a solid piece of pipe. It would take a near nuclear catastrophy to bend or break that post.
And Thirdly...the shim is going to be the way to go if you're set on a different brand of post. Not many companies made posts that long so odds are you're only going to find one in a 27.2 or other popular size.