I "rented" the contemporary jig 20 some years ago when I did my watts linkage. At first I was very happy with the watts solution, my tires were rubbing in the front of the wheel wells under acceleration and the watts linkage solved it completely. I did have one recurring problem, the bolts that attach the links to the center pivot arm would break and let the link drop into the half shaft universal joint. It would bounce on the joint at highway speed looking for a chance to lock up the rear, usually while I was trying to get to the shoulder. After breaking a couple more I laid the geometry out in Autocad and swung the arcs. It became evident why the bolts were breaking. I cut the original mounts off the frame and re-welded them in the new locations. It has been about 10 years and I have not broken a bolt. I did add springs to the links that will pull them up out of the way if they ever do break but I think the problem is fixed. If you do install a watts linkage make sure that the hub carrier if free to move up and down through it complete range before you install the shocks. If it binds in the travel the bolts will eventually break.
I have some pictures on my site that show the old and new watts locations.
http://www.edwinwhite.com/gallery/Cobra?page=1