The repair plate must be welded to good solid steel. These frames were .100 inches thick when new, therefore the repair plate needs to be welded to steel at least .080 inches thick. Mark proposed repair plate with a magic marker. Make a paper template by tracing it over the marked out site. Cut the paper template accurately and trial fit on the repair area.
Paper is much easier to trim than steel.
As in most repairs of this nature, a side plate and bottom plate are required. This method will create a radius shape like the original as well as a bond to whatever original frame corner might be left. Trace the paper template on the steel and cut. Cut the steel with a thin cutting wheel. A few small weld voids will provide for a little frame flex.