When I cut into mine, I was actually surprised that the rust was only on the bottom (mostly) section of the rocker. I could have sliced it where I had good metal, and just replaced the bottom section, but then you have a 6' (?) long weld, and matching the new and old sections up.
Here is where I cut to get to the good metal.
Instead of that. I decided to do the full panel up to the sill. Cutting a straight line, matching the panels up, and welding that long of a seam didn't seem like it was the way to go.
I drilled the bottom spot welds. They are the pain in the @$$ ones.
Drilled out the door sill spot welds, and laid the new panel in place. Trace it on the door posts and cut them leaving an overlap (if thats how you are doing it)
Clean everything up, paint what you can. grind where you will be welding.
I put the new panel on and trimmed every thing up for a good fit.
I had to drill all of the holes for the welds. The new panel didn't have any.
I simply clamped it as I did all of the new spot welds.
Heres where I overlapped the the door posts. Like I said, I was more concerned with strength over beauty. I may go back and fill them and smooth them if I find time.
Once it's covered up, you don't really even notice.
I also sprayed mine with undercoating. Either scuff them first, or maybe primer. Mine is scraping off very easily. It's almost all gone where I hit it with my feet getting in and out of the door.
After I was done, I went to a local undercoating shop and had them spray the insides of the rockers for $35. Then I dumped about a QT of oil into each one for good measure.