For the moment, I'll focus on linkage between the rudder servo and the pair of bellcranks that transfer this movement to the cables. It sure seems to me like the RC car guys must have something that functions almost exactly like this for steering. In terms of commercially available parts (which would be nice in terms of a kit), these car turnbuckles with a clevis on each end might make for a good, no-slop (and adjustable) connection between the bellcranks. Heli-style ball links might also prove useful.
http://www3.towerhobbies.com/cgi-bin...?&I=LXFUG8&P=7
Below is a photo of Tom's solution. Personally, as I mentioned above, I'll like to be able to have a direct link from the servo arm to one of he bellcrank.
The other issue is the direction of the servo arm travel. There are two possibilities: 1) with the arm at 90 degrees to the servo, as with Tom's solution or 2) with the arm lined up with the servo, which might be better for a direct link to the bellcrank. I've already checked and there is room for the arm to move in this direction.
Finally, note that the servo can be "lowered" to whatever height might be best by placing blocks "under" the current mounting beams. This might turn out to be important.