Hi again Mangpong
You have already got a good deal of input from colleagues who are more qualified than myself. I will just add my 2 cents.
In the quest for structural rigidity and strength versus weight there is one theoretical factor that need to be entered in the design. It is the target degree of deflection. Meaning how much do you expect the chassis to twist. In some countries (Australia is one) home built sevens I believe are subjected to a torsion test. A good deal has been written on this in the past, in particular with respect to sevens designed to be equipped with heavier engines like V8s. I got my structural basics from Herb Adams. Chassis engineering. The frame design issues are discussed on page 85.
Here are a few quotes (some of it will of course be obvious to you and others):
The basic shape for constructing rigid structures is the triangle. Its shape and dimensions will not change much unless one of its 3 legs is broken. In contrast a square-shaped set of tubes has very little structural rigidity, in that it will bend diagonally when even a small load is applied to it...etc
Then Adams explains that dividing a rectangle in half creates two triangles achieving rigidity.
And then he talks about double diagonals (and this may be more within your question). He says double diagonals can be used (which creates four triangles) for still more rigidity, but these additional members are usually unnecessary unless very high loads are anticipated.
And back to you, this of course brings the question: how is your car going to be used, what kind of engine, etc.
As you will agree one could go on for a long time discussing chassis engineering. This is a fascinating subject. Good luck and keep us posted.