Need help choosing steel... Race Car Project Answered

You could try it the modern way and use simulation software.
This way you can reduce weight at the same time as you increase strength.
It is a basic bx frame design, so you could focus on the areas of most stress.
For example rounde corners on the connecting angles, done by simply metal disks, increase the sheer strenght, so in many cases a thinner material can be used.
Round steel instead of box shape also gives more stability, so again thinner wall thicknesses could be used, especially i the right type of steel is used.
Of course it makes the preparations and welding a bit harder but is well worth it.
Combined with a pipe bender you might even save on a lot of welds.