First tack weld a brace at the back (marked in red) to keep the roof in position. Cut your Sandman panels out where you have marked and then use a spot weld cutter to remove the remaining panel from your car.

Cut the HZ where I have marked and use a spot weld cutter to seperat the cut sections from the side panels... if you use the spot weld cutter on the cut panels you won't have 6mm holes through your side panels.

Use the cut sections to repair the rust along the top of your rear quarter panels and then fit the cleaned up side panel into position.

Hard to explain on here so if you don't understand what I'm saying let me know.