Did I do something stupid?

The orginal style U clamps are nice, if they have good threads its worth restoring and using them. Chrysler put some effort into their design. They are flat in the U with a waffle pattern, provides the correct clamping and friction on the tube with the book value for the torque you apply to the nuts.
What torque it should be with a plain round U clamp ...? goodness knows.
Be careful not to crimp the axle tube, the force applied by the base of the U is very concentrated on a narrow space with a round section clamp.

Dave