Consider what Eric suggested, that the xls model with the smallest legs retracted acts much like a series 4 Gitzo. Since you may acquire the 400 f/2.8 sometime in the future the xls model would make more sense than any other 3 series Gitzo in that regard.
The 3542xls is the tripod I chose for mountainsides and streams and it works great with my 500 f/4 lens on a Wimberley gimbal. It also works great as my main general purpose tipod with smaller lenses and a ballhead (absolutely rock solid). I do have a series 5 Gitzo for my 600 f/4 because it is substantially heavier than the 500 f/4. I do not consider it to be a problem that I have to adjust the legs on my 3542 xls to something other than full extension.