One technique for using a big lens with a monopod is to keep your left hand as far forward towards the open end of the lens hood as possible. Using your hands at both ends of the body/lens combo gives greater control over the heavy lens swiveling about its pivot point (i.e. the monopod). Doing this allows more control when panning and "fine tuning" the balance point. Keeping the left hand really close to the camera body will have the combo wanting to tilt all over the place thanks to that physics slut, Mrs Inertia.
For a pretty cheap but effective gimbal head try Lensmaster http://www.lensmaster.co.uk/index.htm
I bought their RH-2 and got the shot below last night with my 5D3, 500mm mk2 and Kenko 2x TC. (Yes, I've probably over-sharpened it a bit too much but I leave the good stuff to NASA.)
Of course, you could just do weight training and handhold the 600!
Nice work and not all that oversharpened...what radius did you use? I assume this is a 100% crop? How much better is the Kenko 2x TC, than the Canon series ii 2x TC?