I haven't used the Velbon, but I'd personally lean towards one of the Gitzo GT3xxx series cabron fiber tripods instead. The Z1 may also be a little bit light as well, perhaps the B1, RRS BH-55, or Markins M20 would be a better fit.
I use a Gitzo 1325 and Markins M20 for my 4x5 wood field camera, and I felt the need to upgrad from a smaller tripod (similar to the Velbon). With the Arca Swiss M-Line weighing twice as much, I think you might run into the same problem.