Tube chassis question

[vwbloodline]

To start off I didnt know where to post this so I figured I would try here. I am not familiar with the strength of aluminum box tubing so I am writing in hopes that someone here is. I want to make a tube chassis for my car out of aluminum instead of steel to save weight. My question is would Aluminum 6063-T52 Bare
Square Tube 1.5" x 0.125" prove to be strong enough or will I have to go with a larger size like 2.5"? The design that I am going to go with will be similar to this, however out of aluminum. http://www.thesamba.com/vw/classifie....php?id=615731

And I guess for that matter how much should I brace a Karmann Ghia to support an ej20 t pushing a 915 transmission? I assume a kafer brace minimum. How solid are those of your cars who have done such a swap or something of greater power? Thank you very much for your time alone to read this and especially thank you to those who can help.