It helps if you reach up there with a sawzall and slot the pinch weld a little so that you are only bending back the part you need too. Use a couple of blocks and a large prybar after you get it slotted. Be sure to not go too deep with the sawzall or you will end up with a hole in your floorboard.
I use the wood with porta power to flatten it. Stopping the cuts before the spot welds keeps the seam tight.