I haven't put a panhard bar on one of our trucks, but I've always been a fan of panhard bars over watts links. I've driven both, and while the argument against a panhard is that it pushes the axle side-to-side slightly, you won't ever notice it unless you're a pro driver, and even then it's predictable.
The people selling watts link kits always fail to mention that a watts link doesn't make a straight up and down motion either. It makes a narrow figure 8. They're selling a heavier, more complicated suspension setup that doesn't provide much benefit.
If you go panhard, you want the panhard bar as long as you can make it to minimize the arc of travel. It needs to be parallel to the axle. Typically they're run behind the diff to avoid the pinion, but keep in mind that you'll have to drop one end of the panhard to do any serious work on the pumpkin.
If you know anybody with a S197 Mustang, they have a very nice three-link/panhard setup that you could reference.
