I am working on a 38 but things won't be all that different on a 40. Nothing will be riveted together. If you are replacing say just the front floor pans, I would leave the body on to keep everything lined up. Same thing for the rear area. For the rear I am using panels from
EMS . EMS also makes floor pans but they are flat for hotrod work. For the front floor pans I used
Classic 2 Current panels . They are very authentic looking but not exact. I don't have a bead roller etc. so this was my best option.