Sandblast the snot out of that pinch weld, epoxy prime, use 3M panel adhesive in the groove. Its a 2 part product with self-mixing nozzle.
It sands to a very smooth finish and very solid. When you are done sanding, there should not be much of the product remaining. just enough to fill the gap.
If you address the rust issues properly, this will be a permanent solution. Regular seal sealer would likely not be.
