Aligning tailgate

[E.D.75GMC]

Put them "new" box on my truck and got it all bolted together a while ago, but as the truck isn't being driven currently I never bothered to put the tailgate on until today. I put on all the hardware for it and then put the tailgate itself on, and it wouldn't close because it was hitting the bedside one the passenger side. My dad put a steel pipe in the rear stake pocket and pulled outwards so it could close. Check out the pictures and let me know if there's a way to properly align it without taking the box apart again (it's already been seam sealed). Thanks in advance.