Lay out the steel tubing on the ground, and mark your cuts with a permanent marker. You want to have two lengths that are 46 inches long, and two pieces that are 8 feet long.
Cut the tubing using the chop saw. Layout your pieces on the ground.
Tack weld the pieces into a 4 foot by 8 foot square using the MIG welder. A tack weld is a temporary weld that lets you reposition pieces if need be, and is a short and quick weld. Make sure that the frame is square using the carpenter's square.
Fully weld the frame once it's perfectly square. Weld all sides of the joints, making sure the weld penetrates well.
Mark and cut two 94-inch long lengths of steel.
Weld the 94-inch lengths down the middle of the square, equally. This will provide extra strength for the sheetmetal top.
Place the flatbed frame onto the truck's frame, and center it side to side using the tape measure to check for symmetry.
Using the chop saw, cut the steel plate into small squares that will fit onto the mounts on the truck frame. You want them to be able to mount to the frame as well as the flatbed you're building.
Drill holes in the center of the steel plate squares. You want to progressively step up bit diameters until you've reached 1/4 inch.
Weld the nuts onto the steel squares such that the nut is centered over the 1/4-inch hole.
Bolt the square plate onto the bed frame using the 3/8-inch ratchet and socket set. The bolt will go in through the bottom of the factory mount, and secure to the nut that is welded to the square plate.
Lift the frame onto the truck with the help of an assistant. Make sure to measure it with the tape measure and ensure that it's square to the frame before proceeding to the next step.
Weld the square plates onto the flatbed frame. Weld full beads across all connecting surfaces, ensuring that the mounts will be strong.
Unbolt the flatbed frame from the truck using the 3/8-inch ratchet and socket, and with the help of an assistant, flip it on top of the sheetmetal so that the frame and sheetmetal are square to each other.
Weld the flatbed frame to the sheetmetal using short welds, no longer than 1/2 inch, every 6 inches. You want to do this to the side of each piece of square tubing, the parts that are at a 90-degree angle to each other.
The item is very heavy now, so with an assistant, carefully flip the flatbed frame back over and align it with the truck frame. Secure the flatbed to the truck using the 3/8-inch bolts.