Personally, I'd cage it. Since it's a street car, leave out the door bars or install swing outs. It'll take more time to install it so it won't be too much in the way, but will be much stronger than boxing all that frame......not to mention it may save your bacon if all those cubes get out of hand. If you REALLY don't want a cage, I don't know why you couldn't box most of it with flat plate.....