Classic/Antique Car Repair: 72 chevy truck vibration, vibration source, ball joints

Question
Hi Vern, I have a 72 chevy cheyenne, 350 4bl carb   I noticed a harmonic vibration in the cab as I'm driving on at highway speeds=especially at 59mph. I've had the tires balanced, the drive shaft balanced, motot mounts and transm. mounts all checked ok, and also had it tested on a dyno,  No one can find the source of the vibration. I did notice the problem after I had new poly body bushings  installed, replacing the original rubber ones. The mechanic said that it is not the problem.I'm thinking that poly is too stiff, and the rubber ones would have absorbed more vibrations. Am I overlooking any other vibration source? should I replace the poly mounts?  Thanks, Bob

Answer
Hi Robert:
You have identified almost all the sources of possible vibrations, so I tend
to agree with you. Polymer bushings are stiffer than rubber. However, Have you
had the ball joints checked? Any play in the ball joints would exhibit the
same vibration as a miss-aligned wheel at high speed.