The day has finally come!
First, a massive thanks to Kevin from Red Coral, when I said 'come over on Saturday and we'll plumb the tank!' I fully intended to be more helpful than I was, but I really could have said 'come over on Saturday, and you'll plumb the tank, while I sit around offering suggestions and otherwise being annoying', and still have been correct.
The display is filling with tap water as we speak, but first, here's some photos!
I came last night and spent 2 hours cleaning the entire set-up. There was like a solid inch of construction dust on everything.
Bulkheads go in:
Checking to see that the hose attachment works
All sorts of 2 inch plumbing:
We went with the Reeflo Dart. This thing is whisper quiet
The two overflows needed to be teed together to feed to the gigantic three-way gate we installed.
South end (office and dining room side)
North end (office and dining room side)

We put gate valves on both sides in the hopes that we can make a herbie work. Since the two overflows join in to each other then flow in to a 3 way valve, it might not happen (in which case we'll do Dursos), but here's for leaving your options open!
And the main event - the three way valve. The ones on our pool in California are smaller than this! Everything was positioned to provide maximum access, and to not interfere with the skimmer chamber.
Filling the return chamber for a leak test. One minor adjustment and we're leak free on the return side!
Started filling the sump for real.
This is the part I was most worried about, the water exiting the skimmer chamber has to split in two and fill both the frag chamber and the water change chamber. Turns out it works perfectly
And now to fill the main tank... This is where I'm at now, so far it's been an hour and it's not full!
Next up - testing the overflow system, and that monster 3 way valve...