When fitting panels that have had to be reskinned such as a tailgate skin it is quite normal that these panels are a bit tight or too loose in the original opening. This is over come by slicing along the edge of the opening with a 1 mm cut off disc and then moving the edge to where you want it. This (as shown in the pic ) is welded up again taking great care to keep the gap uniform. Some times the closing panel runs out of adjustment and this means the only option left is to cut the jamb and rework it to suit.