rust reapair near rear vents (half moons)

[super vw]

Im just about to get started on repairing the rust holes around the lower rear window and rear vents (half moon things) and wanted to ask if you guys have any tips or things i should watch out for when replacing the panels in this area.
If you look at the pics you can see that one side it not doing so great, how much of that area should i cut out? should/ can i leave the gutter in place? Or do i have to cut the whole section out arund the rust? i have a donnor 72 super that im getting the metal from. And im using a Hobart 135 MIG welder with .23 solid wire and 25/75 Co2/Argon mix.

so any advice, tips...ect would would be cool. considering im a novice at welding and body work. but i have already welded in floor pans on my 74 super so im not a compleate newbie.

Thanks!
Jonathan