trunk cover repair weld or glue the skin

I have used 3m 8115 to glue many body panels in place on many vehicles. You have to do it on bare metal though. Mating surfaces must be prepared with 80 grit to give it a good tooth. Then you prime afterwards. No adhesive I know of will work well over primer, it needs bare metal. And it will release over 450f, so right above the muffler might not be ideal. I'd strip it again, then spot weld it, then epoxy prime it. You'll have a hard time spot welding through epoxy primer.