Set the u-bolt on a side with a spacer under it so it is not sitting on the threads. Take a hammer and strike the other side, close to the rounded part, (be careful not to damage the threads) to narrow the opening. It really does not take much of a blow, I suggest you do a little at a time.

If the opening has closed up and needs to be opened a little, grasp the u-bolt from one of the sides and strike the closed end against something solid. Again, it really doesn't take much of a blow. Strike lightly.

My shop used to be close to a spring shop that custom bent u-bolts, these words of wisdom comes directly from him, many years ago. Gene