What's the rest of the vehicle look like?
you can buy that piece and cut out the bad, weld in the new.
I was just about to suggest cab corners. It's pretty labor intensive, especially if both sides are shot.
you can buy a crimping tool that goes into an air chisel that will crimp a flange around the piece you cut out. Grind it down, clean all the paint and weld it in. It ain't rocket science, but it is labor intensive
What Sinclair is suggesting.
I've replaced several panels from online ordering before, of course they didn't need welding so it was easier. However, you never ever ever want to plat over rust. You must remove all of the rusted out parts and spray a preventative like POR before tacking and welding the new panel piece in place.
Good luck.
I'd attack that surface rust while I was at it. Rust is like a disease, once it gets started it will spread unless treated.
https://www.google.com/search?q=bes...5.5999j1j7&sourceid=chrome&ie=UTF-8#kpvalbx=1
They don't mention a critical step, which is primer, even on the new body panels.