It does work best to have the control horn centered... But not necessarily required. Just be aware that there will be a differential in turn response. In the direction where the servo horn is of will be less responsive. Say that if the servo horn is off centered to the left, the left turn will be less sensitive. However when turning to the right you'll notice much more tailwheel response. In most cases an adjustment to the linkage will allow you to lengthen or shorten the linkage connections to the control surface.

Both Dubro, Sullivan and Sig make steerable tail wheel kits. Not sure how much fwd you'll need to move your main gear to get the plane to track straight. I'm guess just aft to the leading edge of the wing or about an inch behind.

Good luck and send photo's when you get it done...<br type="_moz" />