First Panel Repair - Rear Slam Panel

SO I pulled the rear slam panel from my 240 and of course there was some underlying rust which I cut out. Not there are alot of complex curves and shapes in that rear section and I have a whole donor rear end. Is it logical to cur down lower to in the panel at a flat area to have a nice mating surface for the two panels. The pictures below may provide some clarity to what I'm saying. Any input as this is my first attempt at THIS much body work ( I have tons more to do on the car).

Here is where I want to cut , I'll pull off the support brace first (already cut out the top rusted sections):

Here is the donor:

I already pulled up the brace on this one and with cut here as well as pull the other spot weld to get it off with the slam panel and relace on the new car.