Rocker under quarter panel

I also agree, that is definitly a structural part of the car. That's where your rear frame rail ties into the inner rocker (where your front leaf spring shackle mounts too) so there will be a lot of stress/force in that area. It looks like you cut past where the rocker goes in behind the quarter, so it might be worth getting an entire outer AMD rocker and completely changing it out IMO. I've already got a set of the AMD rockers and they are really nice thick pieces! If you want to add some rigidity to the car while you have the outer rocker off you can add some extra steel in behind the rocker to tie the torque boxes in (like the convertibles had) if you don't wanna run subframe connectors and have it appear stock.