Have you run those rear disk brackets before?
Most of the ones I've seen have spacers for the bracket or bent brackets, since the bolt is designed to provide clamping force, not shear resistance. Especially grade 8 that you have there.
Simple pipe cut to fit your space would provide more friction in the "clamp". Better would be a solid surface mount on the entire flange.
I am sure that a lot of people have run them this way, but I am also sure GM mounts theirs solidly for a reason after thousands of hours of testing and design modelling.
Sort of like this:
My opinion on the hitch: Do you have bolts running vertically into the piece of metal welded to your frame (I assume that's from the lift gate?). If so, I would run it. Plenty of clamp force and each one of those bolts is rated for more than the truck would tow. Bolting in two planes is always stronger as it can resist more lateral force, which is where most trailers cause problems in side to side sway or worst case, flipping.