You weld?
What kind of machine do you have?
I would consider designing your own platform. I would think raise the new platform up above the wheel wells and out over the wells. Then just use some 1/4" square stock as the bracing between some 3/8"+ angle iron. And make the whole upper platform detachable from the original trailer so it can be easy maintenance.
Just my two cents though. Good luck!
"As I walk through the valley of death I fear no one, for I am the meanest motherf**ker in the valley!"
-General George Patton