When welding the cross rail extension to the trunk drop off does the extension get sandwiched in between the quarter panel and the drop off?
Your tail panel should overlap your crossmember ( be longer) leaving a groove for the trunk drop off to sit in test fit ALL panels using sheet metal screws, when fit is correct weld.... cross member ensuring frame rail spacing is correct per your year tail panel ensuring trunk keyhole is inline with middle of car/ trunk ( divide frame rail measure by 2, mark the spot , its the middle ) quarter panel
