Hey Jennifer,
Welcome to the world of white armor! Can't wait to see how you rock that RS kit!
About that kidney/ab closure... your seam line where they meet on the right should close straight. Like this (using my pics from my EIB application in my RS kit):
I've always been told that the belt isn't structural; you shouldn't necessarily rely on the belt to pull things together. I mean, you can, but this symptom simply means that your kit isn't built properly for your body (yet! ).
From the pic you posted, it looks like your ab/kidney are sitting high on your torso. The top of the ab button box ideally should be positioned at roughly the bottom of your sternum. Maybe share an additional pic or two showing the front? This can alter how the kidney/ab meet up. To be honest, the best way to see how everything fits is to tape the parts together from the inside (back/kidney/butt, chest/ab), then tape the front and back halves together, and try it all on. You will quickly see what needs trimming/shaping, and in what areas.
And your kidney still needs to have flashing trimmed from it. That will also help reduce that gap you're seeing.
Many thanks to @Mworm1974 for sharing one of my build threads with you! However, the one he shared focused mostly on my ATA Hero lid. Take a look at my RS build thread here for much more info about how I assembled my RS kit. I also documented my second build here, but it's a WTF kit (overall, much smaller than the RS suit).
Keep in mind that since you've trimmed the cod, for best fit, you will need to either shorten your kidney or shorten your back plate (or, like I did, shorten a little of both to keep things in good proportion). Looking forward to watching your progress!