I will have to do the same type of repair when I replace the quarters on my 71. AMD no longer makes the 70-1 duster quarters only 72 +. When I make a patch panel I sheet metal screw the new metal over the old then cut both at the same time with the wheel at a 45 degree angle. Remove the sheet metal screw and weld the holes shut. By cutting both pieces at the same time you end up with a perfect patch with a nice tight gap to stitch weld to. The photos are of my buddies blazer I’m working on, I don’t own chebbys. As you are likely aware take your time welding and keep the panel as cool as possible to avoid warping your panel.