The first problem you would face is that Leylandi is very fast growing so in terms of wood hardness it makes Pine look like a hardwood.
The second problem you face is that with only 16" diameter you are going to have an awful lot of heartwood in the planks and it will crack and warp like you can't imagine.
If you do try and turn it into planks it's better to do it sooner rather than later - cut with the grain into 25mm planks and stack them with spacers between the planks somewhere sheltered. You will need to leave it for at least a year before it is remotely usable and perhaps longer. Paint the cut ends of the planks with some sort of paint otherwise the ends will dry out faster than the middle and you will have banana shaped planks.
To be honest, I think you willl get warped and cracked planks anyway so probably not worth all the effort. Better to cut it into logs and sell it for firewood and buy some decent cedar with the proceeds.