Not sure if there is real reason, but all the panhard bars I have ever looked at have been anchored to the frame on the right side. This may have something to do with chassis loading from engine torque, but I can't say for sure.

I have had 2 vehicles with panhards. My T-bucket and my Ram truck. Both have 4-bar rear suspensions. The T-bucket had a very short bar (around 18") and the Ram is much longer (probably 40" or so). On both vehicles I can feel the body floating sideways on the suspension when you go from compressed to extended rapidly. There is a particular road near my house that has some close spaced rolling dips that causes the suspension to move almost through it's full travel if you are going too fast (which I usually am). However, I have never felt it in normal driving, but I do find it annoying.

Keep your bar as long as is practical and level with the ground. Being level keeps the sideways movement the same in both directions of travel. Make the bar heavy duty. I would use at least 1 1/4" tubing with a 1/8" wall. The loading can get high when the bar is in compression. In that situation, only the strength of the material is resisting bending. That 57 will have a lot of load on the bar when cornering, considering the body overhang.

Make the frame brackets strong with diagonal bracing.

Where the bar crosses the centerline of the rear suspension will be the roll center. I would probably try to keep that a little higher than the front roll center, which would be where the leaf springs attach to the axle (centerline of chassis). Keeping the rear roll center higher makes the roll axis run downhill the to front of the car and usually will help give a little understeer, which is way safer than oversteer.

Hope some of this helps.