I really don't see the need to custom fab expensive aluminum when steel tubing will work just fine with only a slightly lower tensile strength per weight. Running triangulated tubing to your suspension members will give the largest increase in torsional rigidity. Try to run the tubing as perpendicular as possible to the mounts so most of the force is loaded in the steel in compression/tension. Putting tubes in bending severely decreases the frames rigidity. I wouldn't worry about using aluminum, especially if you can weld steel yourself. Welding aluminum is not easy, and welding aluminum to steel is even harder.

-Joel