Trim away as much bulk as can. The next step is to drill out the spot welds if you can find em'! This is where the air chisel comes out and helps to locate the fixed points and separate the pan from the brace. Go easy here as you don't want to damage the floor brace. If the welds wont break with a chisel then hammer the pan flat again around the weld and using the grinder you should be able to make them thin enough to break the weld but not grind through the brace. Patience is the key here.