That stand design should work well for a 10 gallon tank, but it isn't likely to work well for a 40 gallon tank. The tank/water weight is carried largely by the nails, in shear. When nails are loaded in shear, they tend to crush the wood fibers, loosening the joint and letting the part being loaded sag. That would then make 4 points on the front and back bottom trim of the tank support the whole tank. (on the tops of the legs.) Also, there is inadequate resistance to racking loads, side to side, unless a back is put on the stand.
Big tanks need to have vertical support under all 4 corners of the tank, all the way from the tank to the floor.