Weld with the fuel tank full, less fumes that way. I welded mine and I could smell gas but I had the body off and covered the tank with a wet towel. Don't worry about drilling holes the rear is the highest part of the frame. The area you are welding is past all the suspension points, I wouldn't worry about warpage especially with 1/8 material. I used a hammer instead of a wire brush then an angle grinder to cut out the pieces, you might be all day with step 1 and 2. Good luck and be patient.