Another friend suggested adhesives, they may work but that panel carries the entire side and back corner of the floor and seat riser. I like the thought of some mechanical fasteners in there doing the actual work. If that panel wasn't so thin I would much preferr to plug weld the new one in but I know what that would turn into so rivets are the best alternative...
The other side of the lower fender is rusted away as well so no help there. I will just round it off and bead roll it, that will look fine.
Mark