Like milk crates? How much do you weigh? Stand on one and see what it does. Jump up and down. Does it give any? I'm sure they could, based on my experience with them. Worked at a grocery store in my younger days. I'd definitely use a board across the top though. You can strap them together with plastic tie wraps and secure the board in the same way.
if you put two of them next to each other with a board on top they do. I have a 10 gallon sitting on that. makes a good little stand and looks pretty cool. Kind of gives it a grungy look, which I don't mind, being a teenager and all. The board I use is about 3/4 of an inch thick, I don't know if that would matter or not, but thought I'd tell you.
In uni, I had my bed sitting on 4 stacks of 4 milk crates... and it was rock solid. Those things are amazingly strong. I would not be surprised if a couple of them would hole a 75gal. (I wouldn't try it though)
I'd say that since each one of them holds 4 gallons of milk and they stack them something like 6 or 8 high in the dairy, they should be plenty tough. Just make sure to use the board to distribute the weight evenly on the base of the aquarium.