Well, you have discovered why 75 gallon tanks, and most if not all tanks with gallon sizes ending in 5: 55, 75, 125, 175, etc. are (no offense) garbage tanks for marine systems. The narrowness (and width to height) is the inherent problem with them, and this causes issues on several fronts, as well as it being hard to get a halfway decent sump under them, when using a properly designed stand.
Personally, if I had to set up this tank, I would build a custom sump that is 17.5" wide, wrangle it in, and continue to march, rather than downgrade to a 20L (which is going to come up.) Then use the 40 breeder at the same level as the DT, set up as a refugium should properly be set up, plumbed to the common sump. Actually though, I would have skipped the 75 and gone for the 120. The 120/40 breeder seem to have been made for each other, aside from custom sumps.
A word of advice however. I see in your signature that you are planning on using "live" sand. There is nothing live about this product, it is nothing but wet aragonite, and the only thing in it aside from water will be anaerobic bacteria, which are the wrong type of bacteria to get your cycle started. The aerobic bacteria for the "cycle" do not survive the packaging and shelf time. It is snake oil, just like every so called cycling aid on the market. Plan on 1 - 2 months to break in your system (cycle is not really the right terminology) whether you use the snake oil or not. I advise you to save your money, and buy dry aragonite sand. The bacteria are ubiquitous and are everywhere EXCEPT inside that bag of wet sand, or bottled cycling aid...
