Not sure if this has discussed but do overtorqued leaf spring-axle housing u-bolts cause a stiff ride?

No.

the stiffness of the springs causes a stiff ride.

The u-bolts holding the axle to the springs would have no bearing on ride quality unless the were so loose that they shifted or fell off...now the bolts out on the shackles on the ends of the leaf springs can be another story. When I was a young lad I helped my buddy replace a set of leaf springs on his car. we put the jack stands under the frame and let the rear suspension "hang". After putting in the new springs we tightened everything down while the rear suspension was still hanging and that car acted all kinds of crazy while driving around town. It took about a month before one of our older friends told us how we screwed up by not reloading the springs with the weight of the car before tightening everything up. Live & Learn!