I just finished a full floor pan ,toe boards, trunk,trunk extentions inner fenders (just patched the lower parts) ,outer fenderwells and full frame rails on my 69. I had the tailpanel out because it was smashed in right up to the rear wheel so the trunk, and floor pan was easy to fit in. For my quarters I cut a parallel line 1 1/2" down from the top body line right to the tail panel, with the same 1 1/2" margin along the door jamb. I used what I believe they call an 80% quarter and cut off all the Jamb area and top body line area(including the area that goes over the top into the trunk gutter). I had cut the new panel within about 1/4" of the body lines. I lined it all up with marks I had made on both new and old panel contours, screwed it on when it measured up correctly. I then cut through both new and old panels at the same time with an air body saw all way around the perimeter. When welding the butts together I used an old saw blade to maintain the small gap and started welding old and new together every 8 inches or so as I cut the two panels. Butting the joint tight without the gap causes buckling of the joint and lots more filler work later. Keep the welds far apart and fill them in between later to prevent heat warpage. This may seem like a lot of work but I have maintained all of the original body line bends and my welds are within 3/8" of the original bends so there is a lot of strength in the joint without warpage from a patch panel style repair where the joint is closer to the center of a flatter panel. The new panels are decent but don't have quite the same radius of bend on some of their body lines. Just my 2 bits.
