[SkidmoreGarage]

I am going through the process of rust repair myself. The best advice I can give is to let the truck tell you what to do. Do small cuts to find where the edge of the rust is, and leave as much original metal as you can. The old stuff is very thick, and can be ground back and still be thicker than the replacement. You will likely be surprised how much rust is hiding, and how much you can just patch with little pieces.

The other side of the coin is to make sure that whatever you remove and weld has sufficient access to do proper rust treatment and priming on the backside. These rockers were designed to rust though from the factory (I just don't think it was a consideration). I would not want to cut where you marked, unless it was required. You may already realize it, but the lower a pillar is probably pretty rough.