I think when i fitted mine i started in the middle, what i mean is found the middle of the splitter and the middle of the bumper and fixed it, NUT and BOLT.
Then worked from there with clamps doing one side at a time, this means you don't end up short or long at the finishing end.
Must admit and extra half inch length wouldn't go a miss to aid fitting them.

Another pair of hands would also be handy.