I use those at work and they are very strong. You can split the unit in half so the tank can sit on top of the system instead of trying to fit it inside the verticals. The main concern I would have is the weak fiberboard that make up the shelves. I would not use that under a tank as it will likely crumble over time and the tank will settle. I would get a 3/4" piece of MDF board from Home Depot to replace any boards you want to use. It might also be a good idea to add a coat of paint on it to make it look better and help seal it more against SW.
My 40B is 1/4" longer than the inside length and width dimensions.