I am not an expert. I would get yours from a specialist nursery and they will tell you. One thing I know now that I did not when I planted them was how to prune the young plants so that they made bushy growth near the bottom. Mine have now been there for 18 years and the trunks are very thick at the bottom and there is about a 12 inch piece of trunk with no growth where cats and any animal can just run through. I have watched other peoples leylandii hedges grow and asked about thickening them up and have been told that snipping off growth in the early years causes more thicker growth to occur, Research this online or ask a professional as this seems an important thing not to overlook, It is much harder to keep the hedge to 6 feet if you have neglected any of this "pruning". Also realise how thick (fat/wide) the hedge will become as these plants are essentially small trees to start with and the hedge may become wider than you realise.