In the latest issue(near the back) of Dirt Wheels there is a snapshot of a front rack for the 400EX. I don't remember who makes it but it looks like a nice fitting rack. You might want to check this out if
you are worried about the front end being too light with a rear rack.

 

I do not have a problem with dragging the six pack rack when I wheelie. A word of caution, if you use it as a wheelie bar, the extra leverage will bend your subframe.

 

I have been using six-pack racks for years on a 250X, and now my 400EX. I have never noticed the front end bening any lighter when I have something in the rack. I wouldn't let this be of any concern.