A good shell is what a lot of us here dreamed of starting with.You are not alone as a lot of us have started with the same if not worse examples of rustbuckets.My car should have been parted out but has made its way back from the dead, which is one more bird surviving.If you got the time and a few extra $ you can do it, although it may not be the easy way.I'm not sure if that part is what is reffered to on other cars as the torque box, but i do know it is an important part of the structure and should be fixed.I think if it is not done the car will sag(as well as fixing the rocker) Keep posting here, you have lots of support and goodluck, you can do it!

David