How far do I need to cut to get to good metal to weld?

Hi folks, I repairing the area just below the tail light on a 69 Dart. I have a little rust out but I am not a body man so I need help knowing how much to cut out. I plan to weld in good steel and use epoxy primer outside and POR 15 inside the trunk and cabin area. Probably under the cabin area also up to the firewall. I will have it on a rotisserie for that in a few days or so.

area.jpg

This is from outside

outside.jpg

This is viewed from in the trunk

inside.jpg