To really add strength, the frame needs to be deeper in the Y-direction. I was wondering if the frame was about the same width as a 2x4 tube, and the tube could be sectioned to match the contour of the bottom of the frame. Extra material in the x-direction will actually do relatively little. If you look at the section modulus (resistance to bending) of a 2x2 solid bar, and a 2x2 tube they are not that different (the solid bar is actually not much stronger in bending). Plates on the sides will add some strength, but you won't get the same bang for the buck as you get from more depth. It is up to you, how to do it, but I suggest more depth. Even if you just added a plate along the bottom of the frame, it would add considerable strength. Look at semi-truck flatbeds; the web in those frame rails is often as thin as 3/16", and the flanges are often no thicker than 7/16", or even 3/8", yet you can put a 60,000 lb concentrated load in the center, and they don't fold. It is material away from the horizontal centerline that will add stiffness. A vertical plate on the side of the frame will add stiffness, but not as efficiently as more depth. If the Isuzu frame is sufficiently stout, a plate would probably work really well. But I have always like the motto: When it doubt, make it stout, out of things you know about.