the front axle being free to pivot in the center, the frame would not
see much torsional load normal to the long axis, unless driven over
very rough terrain.

But when the center pivot freezes up, it puts a LOT of torsional stress
on the frame. My cub Cadet did that, 5 of the 6 welds holding the
front cross member to the main frame members were completely fatigue
cracked. Only thing holding the left and right sides together was the
engine mounts. Mine is an early model, no grease fitting on the pivot
pin. It has one now!!