The original U bolts will likely twist and/or break before the nut would back all the way off. They aren't supposed to be reused anyway. May as well plan to cut the U bolt in half right in the center of the top.
He's right! Save yourself the headache and just cut them off. Go to the parts store in lift and lowering section you'll find the u-bolt's you need! For a little cash you'll have peace of mind with good new bolt's and no headache putting them in!!! Good luck with your project!:burnout: