you might also try cutting the floorpan out of some sort of modern production vehicle at the junkyard. the sheetmetal in recent cars & trucks will be ribbed which will add additional strength that you won't get out of a flat peice of sheet metal.
the floor pans will also be galvanized. this is good in the sense that the floorpans will not rust as easily. but bad in the sense that you'll have to grind off the paint & galvanized coating towards the edges where you'll be welding.