Need quarter panel replacement advice

After re-reading my post I realized there was one other issue I needed input on. The rocker boxes/panels on my car appear to be ok and I'd like to avoid cutting on them if possible. I'd like to separate the quarter panel skin along the seam between the panel and the rocker box. It looks like there is a "beam" or inverted "u-channel" inside the car that sandwiches the lip of the quarter pane between it and the top of the rocker box. I thought about cutting through from the outside with a very thin cutoff wheel, but I don't want to end up having use any filler or do any re-work to the edge of the quarter along the seam. Does anyone have an idea as to what the best way to do this is? Thanks.