The original bed angle or corner strips were spot welded with a spotwelder. Holes in the bed sides or corner strips are not used with a spotwelder. If a spotwelder is not available, some people will drill holes in the bedsides OR corner stripes to approximate the location of the original spot welds and the holes facilitate welding one to another.
I have also seen the corner strips installed by just mounting them to the floor and using fender welting between the corner strips and the bedside as an alternative, although not original.