newbie looking for some answers on best Demon quarter panel

I have seen this problem with NOS quarters if the car is not supported properly while doing the replacement. Did yo try and jack the rear of the rail. Some times the original quarter holds the chassis in place and when removed it goes out of position. Just a thought you may want to check before cutting and welding it up.
I had a demon that was tapped in the rear at sometime and it went out of whack when when removed. My dad always said good carpenters measure twice cut once. Same goes for bodywork .