Buy a tig welding rig, you will also need the body panel adhesives that bond two long pieces of metal, you also did a lot of right angle cuts and gas batti welding, which is called juvenile repair.
If u are thinking you are arthur tussik, lol. Look closely, he manages to cold pull 90% of his work and still hold factory panel gaps, and finish off with spot welding the replacement.
Your rear quarter panel replacement was half assed, the correct cutting point is at the rear window lower edge line, the entire panel is cut/despotted.