I tend to use a scalpel with a brand new blade to cut the excess from around the bodyshell (much better than using scissors!) but keep the wheel arches in place. Then, marry up the bodyshell to the mounts and mark them. Use a reamer to make the holes and set the ride height so that the bottom edge of the b'shell meets the lower edge of the chassis. Then, mark the centre point of the wheel arches (using the centre of the wheel nut within the arch) and use a compass cutter to get a perfectly round wheel arch cutting. I have used this method for many years and find it the easiest way of getting a good result. Hope that helps, cheers, Chris.
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