Options for installing AMD Quarter on a '70 Dart

On the quarter to roof seam, you typically have no choice since they are usually rotted there. The correct way would be to tuck the quarter under the roof and that may require drilling a few of the drip rail spots. Another option is to cut a small slit so the roof can be lifted slightly. Another thing that helps is to trim off some of the flange on the quarter so there is less to tuck. As long as it goes under about 1/2" it will be fine.
I would never section an outer rocker. They are not hard to replace except for the ends and even if you section it you will still have to deal with those unless you just leave the original ends. Keep in mind that is a rust prone area and you have no way to access and seal those butt welds.
Outer wheelhouse are also fairly easy to remove in one piece. It looks daunting at first but once you get into it they come out easy.
Full quarters are the way to go if they are available. The less seams to hide the better.