If that's what SFI recommends, keep it. Better the chassis absorb energy than you.

I've been thinkin over the "green braces." When you make them straight, it really cuts down the angle of support. You might be better off going out, then down and making a couple of triangles. Check the back end of this CE chassis for example.

http://www.chassisengineering.com/chassis.html

You could use chrome moly if you want, but you don't need it. That's why the different wall thicknesses for CM and steel.