When you dry fit the rear quarterpanel,and trunk extension,also bolt the rear valance in place too. This will help to insure that the trunk extension is fitted in the proper location. You will be welding your wheel house to the extension,and proper alignment is critical to the rear valance fitting to the quarterpanel correctly. You don't want to be having to take a cuttoff wheel to your freshly painted car,when you try to bolt the rear valance up,and it dosen't fit. This is an area that requires a lot of massaging on E bodys to insure correct fittment/appearance. It is also the most obvious area that will show if it isn't correct.