There was slight  Contact Pipe to frame but that was corrected at the Shop on Thursday. No difference in Vibration. The Motor and Transmission Mounts are new and Tight. U-Joints are new and pulled Driveshaft and checked to make sure they are not Binding and are Centered. See my last previous Post for what I am thinking of trying next.

  

Check your fan belts for tightness, and see if any accessory pulley is pulling sideways, indicating a bearing failure, or water pump shaft bearing failure.  Sometimes hard to see, especially if you run dual belts.  Joe-JDC

JDC

Mine varied, it would start at about 55mph, smooth out after 65mph and start again at 90mph. This is with 3.89 gears and a T85 trans. When I first started looking for the problem I thought it was possibly the drive shaft, it was custom made, I took it to another shop to check the balance and it was OK, they were the people who suggested I check the bushing.

Thanks Jim. That's much higher MPH than I am having Vibe at. 35-50. Tapers off after 50. I think my next Step is to swap Driveshaft and see if that makes any difference.

  

Good luck, I hope it turns out to be something simple like the drive shaft balance. There are trans shops that can replace the bushing if needed with the trans installed, I did mine the hard way but it was a lot cheaper LOL.

I wanted to Update My Post. I had the Wheels balanced Twice. 2 Different Shops. No change. Had the Driveshaft Straightened and Balanced. No change. So after looking at everything including Brakes and Suspension. II took it to a Shop for a Fresh Perspective. They pointed out Something that I didn't see. If the Engine RPM is gradually increased in neutral the Vibration will occur probably at the same RPM Range as 35-50 MPH.The Problem is in either the Engine or Transmission. It does disappear after 50 MPH when driven. When I had the Motor apart this past Winter I did not do anything other than replace Rear Main Seal and Gaskets. I didn't disturb any of the Internals.Same with Transmission. I did not have this Vibration when I first put it back on the Road. The Harmonic Balancer was Accurate and still is. Could I have a Problem with the Balancer even if its not shifted? On the Plus Side this might give Me an Excuse to rebuild the Engine this Winter? Any Suggestions on what the Problem might actually be? The Car is driveable as is. Just  enough Vibration to be Annoying. The only thing I haven't checked is the Converter to Flex Plate Bolts. Anything is Possible but I am sure I tightened them when I put the Transmission back on the Motor. I will check that Tomorrow.

  

Mark, have you done the latest test with the fan belts off?  You gotta eliminate any belt driven components before you tear the engine down for rebuild and possible re-balance.

John - "The Hoosier Hurricane"

Could the harmonic balancer be the issue

No I have not tried that. Another Job for Tomorrow. Thanks for the Tip. Would a Damper cause this even though it shows no signs of Slippage?

  

Sorry I was thinking out loud did not read all your post