I tack welded the patch to prevent major warping of the quarter panel.
After several hours of tack welding with time in between to allow the metal too cool, this is how it looked.
Then the welds were ground flush with the surrounding metal.
It appears in the picture that the quarter panel is now ready for a skim coat of filler but, unfortunately that was not the case. In addition to some minor warping from the welding, the quarter panel actually curved in a bit toward the trim piece that was removed. Holding a staight edge horizontally on the quarter panel showed that there were places that were as deep as 3/8". I wasn't comfortable using that much filler.
First I attempted to push it out with a portapower but, it just sprung back when the pressure was released. Then my bud Cal tried using a shrinking disk on it while the portapower was applied. That didn't work either. Then I made a couple of relief cuts in it and then welded it back up with the portapower holding the panel out where I wanted it. This helped a lot but the panel still had some low spots around the welds. Cal took care of these with some skiiled application of the Spitznagel. Now the quarters are ready for body filler and they won't have more that 1/16" anywhere when they're done.