What is the intended use? Street? Auto X?
2 x 4 x 1/8 wall is the go to material for most builds. That is what I used for the Astro frame. The rectangular tubing will build a nice stiff chassis without needing a lot of additional bracing other then a good K member. The cage can be built on top to add torsional stiffness. You could also use the original frame and modify that to carry the new suspension pickup points.
Mark