Have you considered that your tub is probably double-walled and not solid back there, and if you cut you will have need to finish the opening with some glass work?
I just glassed up the tailgate opening that my PO had cut. He had done it very crudely. There is wood sandwiched in many areas of the tub, especially around the tailgate area. That wood completely rotted out because he didn't do a good job of finishing the glass.
In my opinion, closing up the back of my tub did not add structural integrity. I'm sure it does add strength, but my tub did not seem to flex abnormally with just a hole back there.
If you had the motivation to check our build thread, start looking around the pages that are in the middle to the end, I have some prety good pics of what you can expect to see once you make the cuts. It could be a good resource for your mod.
Clay's Build Thread
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