the way to prevent warping sheetmetal is to NOT weld 3-4 inches at a time!!! more like 3 or 4 little tacks stitched together, and space those out. i'm sure you won't have too much of a problem warping in those areas, since its the bed and there's a lot of shape, but damn! plus its getting covered anyway, so no biggie. just chiming in...
now that's what i'm talking about! separating the men from the boys right here! not too often do you see beads rolled. that's above and beyond! nice job.