The quarter panel is spot welded to the car around the edges.
Alot of drilling and some cutting involved depending where you want to make the connection and if the replacement panels are "full" or "partial".
I think you're going to hit alot more rust issues than you have hit so far. Get a good look under the rocker panels as well. Anytime I've see a car with damage as you've got, the whole car was usually rotted around all the edges.