If there isn't enough thread on the last u-bolt then you probably tightened down the others too hard. It's like putting on the bolts for your tire. You thread them on, then tighten them, moving around as you go. Otherwise you'll offset the plate in one direction too much.

You can get the bolt on with the gas skid on, you just need to push harder.

I heard of someone else who had the leaf pack bolt inverted as well. Weird.