It is hard to determine if you are talking about the front or rear corners as both attach to the rocker panel. Assuming you are asking about the rear corner I will suggest examining your current inner corner to see if you can replace only the rotted out portion with good metal from the patch panel. No where does it say you have to use the full panel. If you can splice in what you need to the existing inner corner you will not have to break the spot welds to the B pillar. Just make sure you remove all the bad metal so you are not trying to weld to rust.