After a lot of time wedged inside the tank, I managed to remove most of the deep scratches, using my hand and progressively finer sandpaper (400, 600, 1000, 1500 and 2500). Long day! Benjamin's buffer was really useful to even out the surface after the manual scratch removal.
My dad came over today to help me with the plumbing. So grateful to him! First, we put down styrofoam underneath the sump and the display tank, thinking the one inch thickness would compress a huge amount with almost 150 gallons of water sitting on top of it. Amazingly, it did not! So I need to replace the foam underneath the display tank with 1/2 inch, so that the lip of the stand will keep the tank from sliding off during an earthquake.
Because I'm still waiting for the glue-on internal overflow boxes to arrive, we went ahead and drilled and installed bulkheads. Then, I tried to paint the back using the Krylon Fusion. Maybe I was too impatient, resulting in pooling paint that formed bubbles on the surface of the tank. After three coats, I gave up, because we needed to move to the next step: plumbing the drains & the returns.
For the drains, we used 1.5" bulkheads, with a 90 degree elbow in the back, rotated to allow the pipe to angle down toward the sump. Inside the tank, I have street elbows going into adapters that connect to the bulkhead. Around those street elbows & bulkheads, I plan to glue the long-awaited overflow boxes. I hope this makes the drains quieter!
For the return, we modified the previous plumbing to fit inside the sump. Before, the pump was external, causing concern that a pump blockage or failure would be a huge problem. After inserting the check valve (thanks Bryan) and attaching everything to the strong Quiet One pump, I slowly filled the tank with freshwater, to test for leaks. Sure enough, the adjustable valve that came with the system had a slow leak under pressure. Luckily, I had bought another valve last week during the hardware store trip with Sergio & Gus.
Once we cut out the old valve and replaced it, there were no more leaks. I felt guilty for wasting that much tapwater, but the yard is really saturated, and the water actually had a decent amount of PVC flecks and acrylic flecks (and probably some Novus residue) inside of it, so the freshwater test served to clean out the system.
The fish & corals are still alive in their holding tank.