Martin , Martin, Martin . Bear in mind those rear tins are mega bucks to replace and one day your future classic will be worth far more unmolested . May I therefore suggest that you take a look on flea bay , where a couple of different styles of aftermarket fibreglass cut down mudgaurds are available for a fraction of the cost of a new one . Thus enabeling you to keep your precious original in the spares box . If you just cant resist cutting the original , try and incorporate a little shape as opposed to just a straight cut , keep the cut cool to avoid blistering the paint and finish the cut edge with a 6mm plastic chrome trim , again available on flea bay for a neat finish ! Post up some pics when done mate .