Additional progress and some notes (caveats) about the backplate:
So I changed my mind midstream and decided to use the dual MORA3 setup to cool the bench. I set aside the three white radiators I have mounted to a CASELABS SMA8 pedestal for now, as things were just getting too awkward, given my workspace:
Anyway, about that backplate... While it looks nice, my design is flawed because I should have shifted the MP4 further to the rear of the backplate such that the cooling ports on both items were aligned vertically. Had I gone with the bungee cord approach to securing the block to the backplate, the problem wouldn't exist as I would have been able to accomplish that easily. Hard-mounting the block prevents that though and also prevents using the card in the first slot of the motherboard because of the location of the ram sticks.
Thought about bending some hard tubing and going that route, but doing so wouldn't really resolve the placement issues and figured why bother now.
...consider it a durr moment, one of my better ones. But hey, the backplate was my first backplate, a prototype afterall (when in doubt, rationalize), and will do until the Optimum PC active block arrives. At least I was fortunate enough that I had the parts on hand to rig a connection between the backplate and the block.
Next up: prep for leak testing and adjustments if needed. I'll use the dual pump setup that's mounted to the lower left of the bench frame to do that, then finish the two loops.