object on a spline following another object(superspline)

So I have two objects, a sphere, placed along a spline(using superspline) and a box, which is placed just beside the sphere. I want to set the sphere's position.z to equal box's position.Z while keeping its x and y values along the spline. The problem, at least what I perceive to be the problem, is that in the superspline examples, a single variable moves the sphere along the spline, and I can't seem to figure out how to separate the z position of the sphere from the spline so it could follow the box as it curves with the spline..

Here's an image of what I'm talking about, done in 3ds max:

http://i.imgur.com/KOubmSi.jpg

Note, it's important that the z position of the sphere and the box are the same. Any help on how to approach this would be great. Thanks!