If the stand flexes too much as
@Baluncore discussed, it will put uneven pressure on the base of the tank.
If this happens to glass, it could crack. Plastic tanks would be less likely to do that. Some glass tanks have plastic frames which might counteract this to some extent.
Typically stands are supported at the corners of the tank, and sometimes in between also (like what
@berkeman has shown). This would reduce the possibility of the stand flexing. This makes a more clunkier looking stand but it is stronger.
I had a friend who made tank stands from metal, like you are proposing. He would look up in a engineering book how much the square tube metal he was using would flex (there are tables). As
@Baluncore said supporting the load across the long dimension of the tube will result in less flex. You might be able to get similar information from where ever you are getting your metal. You might be able to google it up.
It might also be worth considering the kind of metal you intent to use (this will also affect its strength properties). In the fish facilities I ran, we would always use stainless steel. Otherwise you could have corrosion and rusting issues (which would be bad looking and weaken the rack).
In addition to the wood racks in
@berkeman's pictures there are also commerically available, slot together mini-warehouse type racks that can work quite well for aquariums. Smaller than the warehouse racks that can hold pallets, but very strong and don't require welding. They are also weight rated.
It is also "traditional" to put a thin sheet of styrofoam under the tank and on top of the wood shelf. This is too even out any small bumps and spread the load.
I did not calculate you volumes (based on your dimensions) and don't know if you added in the weight of the water when the tank is filled. This important since the water will weight more than the tank itself. Water is a bit less that 10 pounds/gallon, if I recall correctly.
Another weight consideration is what will the tank be sitting on and what are the size of its feet. All that weight will be supported by (possibly) a few square inches of surface. This might go right though a floor. some people try to align the feet with joists under a floor. Optimal floor would be a cement slab. Wood floors might even get dented. In some cases you might want to but a thick sheet of plywood under the stand to spread the weight over a larger area of the floor.