It would be great if you could post pictures of the quarter you are replacing. Bracing is important as is supporting the car on the suspension while doing this job. I would place the rear end housing on stands and make sure the body is not twisted out of shape. Do as Dave suggested; don't move the car after this work is started. Use as much of the factory seams along the perimeter of the quarter if the damage is extensive. I even replace the door jamb area so that I can use the factory seam there too. I would use the trunk opening factory seam also. Make the non factory seam area as small as possible. Flanging the new seam area will add strength. A tigged butt weld will give the cleanest job here, but is more difficult to do. After you have braced everything and start cutting away the quarter, take extra care not to damage the panels the quarter will weld to any more than is necessary to remove spot welds. This pays big dividends in the end. Hope this helps.