[Alan Roehrich]

If this car has the stock suspension, un-altered, other than urethane bushings, and the rear end housing has not been changed, damaged, or otherwise altered, if the frame is not bent or broken, then the pinion angle should be close to stock. Unless the ride height is significantly altered from stock.

Now, if it has traction bars, no hop bars, adjustable control arms, the housing has been swapped or altered, or the ride height has changed significantly, then the pinion angle could be off.

If the pinion angle was off enough to break something, there should be a vibration present. Of course, solid motor mounts bring about a vibration of their own, and that could mask any vibration brought about by another cause. <------- Another reason that I despise solid motor mounts.

If the dowel pins do not protrude into the case far enough that the chamfer on the pins is well past the chamfer on the case, so that the full diameter of the dowel pins is well into the holes in the case, that could cause the problem.

At static ride height, the slip yoke should be about 1" out from being fully bottomed in the extension housing.

I think the first thing I'd do is verify that there are no problems with the rear suspension or rear end housing, and that the driveshaft is correct, in length and in tubing diameter and thickness, as well as having good u-joints. After that, I'd put the mounts back to the stock configuration, either new high quality rubber mounts, or new urethane mounts, on the engine and transmission. If you want, you can fabricate a torque limiter of some sort. I'm betting at that point your case breakage problems will cease.