The only JR servo that specifically cannot be run on 6v is the 341 if I remember correctly. I run everything I own on 6v without issue.

JSF-TC - How long is the control horn going to be on the elevator itself? Here's some info you might find useful.

This is based upon a 2" chord, 10" span control surface @ 200mph with the control horn 1" from the hinge line (not the surface of the elevator).
1/2" servo arm, 1" control horn - Servo travel of 45 degrees gives 20 degrees of surface movement and require 25oz" of torque.
3/4" servo arm, 1" control horn - Servo travel of 45 degrees gives 31 degrees of surface movement but now requires almost 60oz" of torque.

So any of your servo choice SHOULD work ok. Of the 3 servos you mention, I'd prefer the DS368 due to the metal gears.