[hemi43]

You are missing the bar that goes between the stop light lens and the reverse lens. For an easy fix, mount your reverse light and stop light and then use the factory trim piece around them so you won't need that bar.
You will need to remove those tabs or just bend them out of the way because that lens housing shown in your picture needs to sit 1/2" deeper.
If you use the factory trim rings like I suggested, you will need to drill it's mounting holes in the body, but if it was me, I would glue them on with windshield urethane or construction adhesive.

I didn't want to use those trim rings, but I messed up and forgot to fill the mounting holes when doing the body work so now I'm stuck with them.
FWIW, when these truck came from the factory with those trim rings, they also had 2 pieces of trim that ran along the width of the tailgate.