Wow, that surely stinks. I wonder what can be done sensibly to bring up the GVWR....higher rated springs? Make sure your tires are rated for your actual load. If your frame isn't rated to handle the load, then you're in deep do-do. Make sure your rig isn't coming apart at the seams, an indication the frame is inadequate.
I used to know this stuff fairly well but I recall the gauge thickness of the frame of an E450 is significantly thicker than that of an E350. But ratings within a particular chassis like the E350 for example, are rated at 12,500 11,500, 10,700 and 10,000 of which I believe some of the weight ratings are determined by the springs Ford installed. Other factors are the engine, fuel tank, and wheel base. It seems to me that smaller ones weigh less so maybe you could load more on the chassis with the difference in weight. There are a lot of dynamics going on with ratings. It's not just about weight. Some of it is about capability. So a smaller V8 Triton engine may rate the GVWR downward even though it is lighter than the V10.
I recommend you call
Champion Frame Align and pick their brain over the phone. I always appreciated working with George but he might have recently retired. They might be able to offer some practical solutions. One of Champion's specialty is just this kind of thing.
Oh.....I see ScottG beat me to some ideas.
