If a bolt is too long, you can use a cordless drill and a hacksaw to cut it down to the exact length you need.
Insert the bolt in the chuck of a drill and tighten.
Now, let the power of the drill do the work as you hold the hacksaw in place.
Another tip is to slide the nut on before you cut the bolt, since the cut may be rough and make it difficult to get the nut on the bolt.
Taking the nut off also helps recut the threads of the bolt.