Earlier in this thread I mentioned frame boxing the middle open rails which would greatly stiffen the frame. Probably more than anything else. Hellwig sells frame boxing kits for G bodies. However, frame boxing is not an DIY project, at least not an easy one. To do it right the frame most be stripped of the body, powertrain, and suspension and clamped into a chassis jig to insure it is square before you box it. The frame is flimsy and easy to flex out of square without a frame jig. You don't want to box the frame while its flexed out of square. Then there the matter of removing the factory wax coating around the weld area.
Even with a jig, its still easy to badly warp a frame from all the welding without carefull heat control. Many DIYs have F***ed up their frames attempting to box them. Back step welding should be used to reduce heat warpage. After that the frame should be measured again for squarness and pulled straight again if need be which is likely. Yeah there are lots of Youtube videos of DIY guys boxing their frames just on shimed jackstands and even resting on wheels, and most are either crooked or heat warped without the use of a jig. It doesn't matter how strong a frame is if its twisted.
It is possible to construct your own frame jig but again its not an easy DIY project. A frame jig must be large, accurate, rigid, heavy, and square. Overall, frame boxing is best left to a professional frame shop as they would still be cheaper than a mediacore homebuilt jig you may only use once. A jig made out of wood won't work either as wood is never straight and it moves with drying and humidity changes.
Simply welding plates to the open C channels does little to imprve the strength of the chassis, it will increase torsional stiffness a little. The better method is to weld another C channel to the middle C channel similar to old convertible frame construction. Boxing the frame this way will nearly double the width of the middle rails and greatly increase the moment of interia in the middle section of the chassis.
Some suggest adding roll cages to stiffen G bodies, which they due to some extent but their main purpose it to be a safety device for track environments but not street environments. But they have several practicality, safety, and legality issues. Basically roll cages are unsafe for street use as they require the driver to use a 5 point harness and helmet. Some states forbid roll cages and helmets on public roads and most racing 5 point harnesses are not DOT approved.
