1. Loosen all the mounting bolts, not fully but enough that allows you to move the tail light.
2. Get an extra set of hands. This can be a friend (male/female), your wife/significant other, partner, it’s an equal opportunity situation as long as they have 2 arms and 2 hands.
3. Have said friend “helper” hold the tail light as close to the body of the truck while you tighten the mounting bolts.
That’s as good as that’s going to get.
This also may not result in what you want as some mounting holes will just “go back” / center up to the location you have now as you tighten them.
