Would you care to share what tripod and tripod head you are going to use it with. I have settled on the Wimberley WH-2 - Version II and am looking at one of the Gitzo Systematic Series 3 tripods - maybe the GT3532LS or the GT3542LS. I have also looked at the Real Right Stuff Series 3 - 34L. Problem is that Real Right Stuff only sell through their own shop, so I can't really look at them with out ordering one, whereas with the Gitzos, I can at least look at them at my local camera store.
I use a Really Right Stuff TVC-33 and the PG-02 LLR side mount gimbal. Works great. RRS gear is really top notch, you can't go wrong even sight unseen.
Compared to the Gitzo GT3532LS and GT3542LS, the RRS TVC-33 is >20cm taller when extended, 1-2 cm shorter when collapsed, and a few tens of grams lighter as well. The RRS TVC-34L further adds another 30cm in height, and is even shorter when collapsed. Overall better specs than the Gitzo 3-series.
Unless you really
need the extra height (the TVC-34L provides it, although the Gitzo GT3542LS doesn't), I'd go with a 3-section tripod over a 4-section tripod. You want maximum stability for the 600mm lens (I have a Manfrotto 4-section CF tripod for travel with 'normal' lenses up to 100-400, although I may replace that with the RRS TQC-14 at some point, both fit in carryon luggage).
The RRS gimbal is excellent. I debated over the full gimbal vs. the side gimbal, and chose the side gimbal (RRS recommends that over the full, actually). It works great! One advantage to the RRS gimbals is that with an additional $140 purchase, they can convert into full multi-row pano heads for your other lenses, if you want to shoot panos.
Regardless of whether you go with the RRS or Gitzo/Wimberley head, I'd definitely get the RRS replacement lens foot. It extends further back than the Canon foot with a Wimberley P-50 installed, and that extra extension is needed to balance the lens on the gimbal if you're using a TC with the 600 II (I have no idea why Canon designed the lens so the tripod foot is forward of the center of gravity with a 1-series body attached, but they did!).
Hope that helps, and hope you get your lens soon!!