I'm going to start calling my truck the Grey Ghost because it's... uh... primer grey and was headed for the scrap yard when I bought it.
While I'm waiting for Gary's crate motor to arrive from the great northwest, I decided to tackle the rotted driver's cab corner.
I removed the fender (five or six broken bolts):
… and here's the corner. Yuck.
The corner support was completely packed. I can only assume it was silt from a creek. It's not rotted, so that's good.
Next step, trim the edges, make templates, make patches, paint the back sides, weld them in. Nothing to it.