So, an update. I have more pics I need to load, and maybe some video. To Channing's question, the notches would have created air pockets, at least on one side. Since the capacity on these was only 14 gallons, and less than that since I chose to drain out of the side instead of the bottom, and couldn't get the fitting low enough, I didn't want to jeopardize anything. I think the floor is actually siff enough. I only cut 2 of the ribs, and replaced it with a abs box, so I think it may be a net zero on the rigidity.
The tanks are connected to the lines with rubber couplings. That adds some flex, made it easy to drop the tanks, and the black rubber will never see the light of day, so it should last pretty long.
Here's a huge tip if you want to build your own tanks. Use lots of ABS cement. Glue all the flat pieces, then run the abs welds on them. I had no leaks where I joined the boxes with cement and ran a bead inside and out, then slathered on the cement. The problems were the tops actually. It was a heat weld, and then an abs slurry on the outside. It just didn't hold. Both tanks leaked when they were filled to capacity as the water pressure broke the weak bond. I think the heat bond is much weaker than the chemical bond.
So, I took both tanks down, and then I put rubber caps on the drain sections, and put my shop vac with a tapered nozzle into the top where the vent goes in and hooked it up to the blower side. It is a LOT of pressure, more so than the water. It ballooned the tank up quite a bit, possibly expanding the middle from 4 inches to 8 under pressure. I could feel the broken seams just blowing water at me. Then, I put it on suction, That collapsed the tank and opened up those broken seams. I then slathered those seams 3 or 4 times with the cement while on suction. that sucked the chemical inside the seam. I then turned off the vacuum, and the crack closed up. I let that cure, and rechecked, did it again. Then I reversed the vacuum to blower and did a soap bubble test. Found a couple pin hole leaks, and then finally, completely air tight at the highest sustained pressure.
As far as tank supports, I ran some hurricane banding under the tanks, holding it to the floor with elevator bolts that were epoxied in from the top, but recessed to hide under the new tile foor. Even full of water, supported at every 12 inches, the 1/4" black abs barely flexed.