You have to re-drill the mounting holes to bring the plinth inside the edge of the surround, there are a few people who have carefully trimmed the plinth so it fits over the surround but I was happy to drill the bodywork.
Have you re-drilled the real holes as well? You have to unless you don't mind the stop/tail lights overlapping the edge of the bodywork (that looks a bit naff in my opinion). How many lights have you got on the rear, I find the 8-light set-up is more effective and just looks better than the 6-light set-up?