I was able to see the photo by copying the link. It looks like you took the metal off the outside fender skin and see part of the wheel well such as #65636A behind it. Welcome to Hell?

I went and looked at this part of the truck and it looks to me like this inside-wheel-well and the outer fender skin are spot welded to each other inside the flange that bends in around the lip there. This joint serves to hold the fender skin in place and increase its rigidity but I think you can live without a few inches of it.

You can patch the fender with fibreglass and build the surface with bondo but a lasting repair will require that you eliminate all the corrosion from the inner piece of metal too. Otherwise, the wheel well will continue to rust.

This is a lot of work, but I would consider to cut out the rusty inner wheel well and have a clean piece of galv steel welded in there (why not do the fender this way too if you are going to go this far...)

But you can probably do a decent job by removing what you can see/reach of the rust and treating the remainder with chemicals. Inside this cavity you really want to let your paint/chemicals dry well. I would not be opposed to a wax+oil cavity sealer like the one Eastwood sells in a spray can in this area before closing it up (watch for drains and dont clog them.)

If you are going to clean and undercoat the wheel well area (you don't have to remove all the old undercoat, just clean it and sand coarse) you can do a destructive test for rust with an icepick. Anything that punches thru must be removed. The pick will pierce the undercoating wherever you poke it, so you must clean the area and reapply it if you do this.

Don't forget to feather out an inch around the repair before you paint too.

About bondo: Bondo is to build surfaces, not fill voids. Bondo holds water like a sponge and promotes corrosion.