If you can reuse the roll from your sides. Cut the top of the bed about an inch below the bend and get sheet metal cut with an offset bent along the top. Weld it in with the offset to the outside from the inside of the bed. I am only welding about a 4" wide strip in where the bed side rails attached and rusted out. I will butt weld this but the offset may help reienforce the top. Rolling your own would be a real chore and probablly not something that I would try. Weld slow short beads. I like to tack each end of a long ewld then the center then the center of of the two halves then the center of the four quarters etc... Then you have to deal with the bottom.
Good luck
