I add extra sheet metal to the floors of ALL of my cars, whether they are rusty or not. Wedges are seriously weak in that spot, and most converts have rust there from leaking rain water gathering in that spot. If you go to a high HP car, you will rip the floor anyway. Start by removing all of the undercoating from the bottom, and the seam sealer and paint from the inside. Use sheet metal that is thicker than the metal that is there. My favorite is sheetmetal gathered from roofs and deck lids of parted out cars. It's thick and usualy not rusted on parts cars. I weld in several angled pieces on the inside of the car. They should be attached to the floor and to the hump in the floor at the rear. Weld in pieces on both sides of the seat platform that extends forward. Under the car, start by welding the seams under the lower control arm attachment points. Weld in pieces next to the attachment ears. Take a large piece of sheetmetal(something like 8" by 10") and attach on the bottom. Have the rear edge follow the rear seam. Use a hammer to make the piece contour to the bottom of the floor. I like to drill several holes in the piece for spot welds as well as a perimeter weld all around it. Once the welding is done, grind smooth the welds and paint with a good chassis paint, and a rubberized undercoating. One more thing I like to do is weld grade 8 fender washers over the control arm mounting holes. The factory holes tend to oblong over time, which will lead to the car pulling left and right when you hit the gas and let off. Should take several hours to do both sides, but it is worth the effort. Make sure someone is around with a fire extinguisher. I once did a car where the carpet I pulled away, caught on fire when it flopped back over the spot I was welding.