Doviiman had FTD build him an 800 gallon tank. He was happy with it.
https://www.monsterfishkeepers.com/...-822-gallon-ext-aquarium-build-thread.442108/
They built my 15' x 4' x 4' tank. I'm happy with the tank. Unfortunately I had an unscrupulous salesman who worked remotely..he had just left Tenecor which went out of business. The salesman is long gone, The owner of FTD and the tank builder are really nice fellows. If I needed a new tank, I wouldn't hesitate to have FTD build me another tank. FTD built the tank exactly as I asked them to, adding eleven holes on the top of the tank to accommodate my mechanical filtration. I changed the diameter of the holes after I placed my order. It wasn't a problem for them. FTD can build a tank to your specifications.