For clearance on the bottom tank the highest part that sticks up is my lid when open all the way so I took that and added 4 inches so I have plenty of working room. The stand is also deep enough so that the hob
filter is inside the stand. As for racking the 2x4's holding the second self up will prevent the side to side motion. My gf has a cat that likes looking at my tank and hitting it. Even though I yell at him. So around the bottom tank I will have a thin acrylic sheet on front and both sides that will be on hinges and have a latch to keep the cat off the tank and allow access to all sides.