I agree with JakeBrake. Do not use a panhard bar with leaf springs.
A panhard bar pushes the axle left and right, because it is fixed to the frame and rotates around that point. Panhard bars are typically used on coil spring suspensions, and tranverse leaf applications.
A watts link would be better, but still shouldn't be needed on a parallel leaf spring application.
Why are you wanting to install a panhard bar?