I am coming to the conclusion that it probably is an inferior part installed in my right suspension. The left side does not make noise and neither does my other '66 which also had its front end rebuilt. So I think the right side needs to come apart again.
As far as urethane bushings go, I would be careful since they are hard and I doubt were even around when Ford was engineering the Mustang, so no consideration would have been made for them. I did install them in a 1980's pocket rocket I have of another brand and they really tightened up the handling. Nice and crisp, but I had the knowledge that these were used in these cars in rally events back then, so I knew they would work to improve the handling considerably. Of course for Mustangs they are not Concours and they usually come in bright colors like red and blue so they are easily seen.