Mine gets wobbly with 7 or 8 gallons in the keg kettle. Not scared here, but I bet a full keg would get it wobbly enough to concern me a bit. So 50 gallons seems like a lot.
I struggle to think you'll find a non-commercial one that's solid under that kind of weight. I'd have to think you are pushing 500 lb at that point. You probably would need to have some customization occur. If you can't find a welder, maybe buy a burner with no stand and build something out of steel pipe.