
I had that same rust spot, cut it out, put in a steel plate, filled with some bondo, and slapped some herculiner on both sides from the trim down. Easy fix, and looks good I think.
If I keep it, I think I'm going to have a tube bumper bent up that fits in that spot and just cut the rear quarters below the trim line on both sides. I think a vendor here used to sell one like that, but I'm not 100% certain.