rhythmic vibration

I'm having vibration issues with my Barracuda. The car was originally a '67, 273/904. When I bought it 3 years ago, it had a hipo273/904 and ran with no vibration. This past winter, we put in a rebuilt '70 340, 727, headers, drive shaft, and an 8 3/4 rearend. The car vibrated horribly the first time out of the garage and had some noise. We checked compression and it's good in every cylinder - 150-160. Most of the vibration worked itself out, until it setttled into what I call a "rythmic vibration". It felt as if I was driving over mild rumble strips. It was irritating, but livable. The first thing we attacked was the drive shaft. I had it cut down and rewelded in Titusville, PA. I called the guy and he said he has no way of making sure it's balanced. I took it up to Erie Drivetrain, in Erie, PA. They checked it out and found it 110 thousandth's off. He recut and welded it, and added a small weight. We put things back together. The noise was gone, the vibration remains. Actually, shortly after this the vibration grew worse. It got to where the steering wheel was vibrating. I was running 323 gears in a 489 case. I had a set of 294's in a 741 case, so we swapped center sections. The vibration disappeared, for awhile. I first noticed it again on decelleration. Within 100 miles, the vibration is back fulltime. We are puzzled. As part of my overall upgrade, we also put in green axle bearing from Randy's Ring and Pinion. I called Randy's, and they said I need to either take the "buttons" off the center section, or have my axles ground.

Additional info - When going down the road, I shift into neutral, and the vibration goes away. When we had the car on the lift, with the rearend up in the air, I revved the engine in neutral, and the rhythmic vibration is present. When driving, the vibration changes according to rpms.

Our plan is to grind the axles. Has anyone ever encountered this before? A little bit of direction would be greatly appreciated.
Author: admin