Jeff,
It all depends on how extensive the rust is. If it goes all the way up on the sides of the foot well, it would be best to replace with preformed panels, but if it's just the floor portion, a regular piece of 18 gauge from the hardware store can do the job just as well. If you want to shape the ribbing into the panel, get a piece of hard wood and route out the shape and pound the sheet metal with a small ball-peen hammer - many light taps. If you don't want to weld, pop rivets work just as well. In that case, put a bead of sealant - like the black 3M window seal that looks like rolled up, soft licorice - between the rim of the remaining floor and the overlapping new metal. If you go the pop-rivet route, you can use galvanized sheet metal. Plenty of under-coating bottom and top, and you will hardly notice the repair.
Bob S.