The rust is greater than you see, sort of like the tip of the iceberg. You need to cut out all the rust or it will continue to eat away at the post. You can not hide it with bondo. You will have to wait till after you cut out all the rust back to good metal to answer your question. If it is only minor, some patches made out of good metal ( possibly from a donor truck), and a little fiberglass, sanding, and painting should do. If it turns out to be a big hole then get some preformed replacement metal form someone like Jeff's Bronco Graveyard or LMC Truck.