Work on one pan at a time. Drill out the spot welds, take the old pan out, put the new pan in, weld it, seam seal it and paint it. When you spot weld the new pan in, weld a spot, then move to the other side and weld a spot, then at each end. Now go back and spot weld every 3 or 4 inches, then go back and fill in the blank spaces. That way you won't warp the pan. Now, do the other side. Check the hump and toe boards carefull while you have it torn down. If they have some rust on them, you might want to use POR 15 or some other rust inhibitor/encapsulator on the rust before painting them.