If the side bars on the old shackle weren't offset, the new one should go back in. If the spring isn't offset too far, you could use a come-along to move it to line up the shackles.
If you do loosen the u-bolts, it's recommended to replace them. You don't want a spring u-bolt to fail going down the road.
When I installed new springs and shackles, I needed to persuade the springs a bit to get the center bolts into the spring seat holes before installing the u-bolts. I think that issue is fairly common.
