Okay based on earlier description of the motion ratio formula:

Zilla:

mr=(14/6.5)^2
mr=(2.15)^2
mr=4.6

YFZ:
mr=(14.5/9)^2
mr=(1.61)^2
mr=2.595

Okay best way to fit the yfz shock on the zilla:

Move upper Shock mount hole: .5" upward
Move lower shock mount on a arm: 2" outboard (closer to ball joint) (or 8.5" from pivot point on lower a arm)

This gives us a new motion ratio of:

mr=(14/8.5)^2
mr= 2.71

This will provide a tad softer ride than the yfz but should not be noticeable or require a change of springs. If you keep the lower a arm mount at its current location the leverage against the shock is too great for the yfz shock to be effective and provide the kind of performance it is capable of. This requires a little more work to make right but this is not a hard modification to make on either the a arms or the upper mount. Let me know if you have any other questions.