Build A Faster Car - helping drivers engineer

I am interested in others opinions on what is the best crossmember structure design to add torsional stiffness to a ladder style frame with the minimum amount of mass (weight) being added.  On past projects I have had good results with 1 1/2" X .120 wall round tubing in an X configuration extending from the rear suspension pick-up point to as close to the front suspension crossmember as possible.  I would then add a lateral crossmember from near the intersection of the X to the side rails. 

Are there better options without adding any frame structure above the frame rails (no roll cage)?  I don't have access to computer modeling software for testing at this point.  I would also like to hear what software folks are using for modeling this type of project.

Thanks