Another drivetrain vibration.

I am trying to get my drivetrain smoothed out a bit. First, what I have. 1973 scamp ,512 ci, rebuilt 727 trans (by me), narrowed 8 3/4 rear end, 741 with 3.91 gears (bought here), front end completely rebuilt from drdiff parts, new rear axles from drdiff, new driveshaft from drdiff.

What has changed, I ordered the wrong u-joint for the driveshaft so I got the correct one from drdiff, BUT, I did not have the driveshaft rebalanced after installing it.

We welded on a back brace on the rear axle housing, Could it be warped. When assembling everything was perfectly straight.

I have a Prothane, urethane rear transmission mount. Could it be just transmitting more "normal"sensations?

I was down 9* on the pinion angle and I placed a 6* wedge shim in the rear perches.

What I am experiencing is that it seems less under mild to hard acceleration, but at 65-70 mph there is a harmonic surging vibration. Hit the throttle and it goes away. While cruising at 65 without the throttle and you shift into neutral it coasts smoothly as can be.

How can I track this down? What other things can I do or check? Rebalance the driveshaft? Try 4* shims instead of 6*? Thoughts??? Help, please.